Use a python3 script to call python3

It's odd, but because Windows requires a full Python3 install we must
have an executable called "tools/python3/python3" to use Python3 in the
toolchain.

Before, we simply symlinked to /usr/bin/python3 (for Linux) or
/usr/local/bin/python3 (Mac).  Unfortunately, depending on the method of
installation, on MacOS the Python3 executable can be in /usr/bin/python3
instead.

To avoid the entire issue, unify the Mac and Linux python3 placeholders
to use python3 itself to jump to the real executable.

Fixes esp8266#6931

Author: earlephilhower

package/package_esp8266com_index.template.json

@@ -156,38 +156,38 @@
                   },
                   {
                      "host": "aarch64-linux-gnu",
-                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-placeholder.tar.gz",
-                     "archiveFileName": "python3-placeholder.tar.gz",
-                     "checksum": "SHA-256:d8cf9d9d66423d7b90978ebe285a73a6e8611995cd0d5e6273e929a0cf2c9850",
-                     "size": "191"
+                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-via-env.tar.gz",
+                     "archiveFileName": "python3-via-env.tar.gz",
+                     "checksum": "SHA-256:c9237bfe0f62842d7187a39495baa4a7e3ab8b87c0b433614294b023cf0bc0f3",
+                     "size": "292"
                   },
                   {
                      "host": "arm-linux-gnueabihf",
-                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-placeholder.tar.gz",
-                     "archiveFileName": "python3-placeholder.tar.gz",
-                     "checksum": "SHA-256:d8cf9d9d66423d7b90978ebe285a73a6e8611995cd0d5e6273e929a0cf2c9850",
-                     "size": "191"
+                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-via-env.tar.gz",
+                     "archiveFileName": "python3-via-env.tar.gz",
+                     "checksum": "SHA-256:c9237bfe0f62842d7187a39495baa4a7e3ab8b87c0b433614294b023cf0bc0f3",
+                     "size": "292"
                   },
                   {
                      "host": "i686-pc-linux-gnu",
-                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-placeholder.tar.gz",
-                     "archiveFileName": "python3-placeholder.tar.gz",
-                     "checksum": "SHA-256:d8cf9d9d66423d7b90978ebe285a73a6e8611995cd0d5e6273e929a0cf2c9850",
-                     "size": "191"
+                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-via-env.tar.gz",
+                     "archiveFileName": "python3-via-env.tar.gz",
+                     "checksum": "SHA-256:c9237bfe0f62842d7187a39495baa4a7e3ab8b87c0b433614294b023cf0bc0f3",
+                     "size": "292"
                   },
                   {
                      "host": "x86_64-apple-darwin",
-                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-macosx-placeholder.tar.gz",
-                     "archiveFileName": "python3-macosx-placeholder.tar.gz",
-                     "checksum": "SHA-256:5bfa0d4c2dc3edeeaa913f4eac42ef3ff0bf8c8fe9f11be112a8ca7911de2dae",
-                     "size": "198"
+                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-via-env.tar.gz",
+                     "archiveFileName": "python3-via-env.tar.gz",
+                     "checksum": "SHA-256:c9237bfe0f62842d7187a39495baa4a7e3ab8b87c0b433614294b023cf0bc0f3",
+                     "size": "292"
                   },
                   {
                      "host": "x86_64-pc-linux-gnu",
-                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-placeholder.tar.gz",
-                     "archiveFileName": "python3-placeholder.tar.gz",
-                     "checksum": "SHA-256:d8cf9d9d66423d7b90978ebe285a73a6e8611995cd0d5e6273e929a0cf2c9850",
-                     "size": "191"
+                     "url": "https://github.com/earlephilhower/esp-quick-toolchain/releases/download/2.5.0-4/python3-via-env.tar.gz",
+                     "archiveFileName": "python3-via-env.tar.gz",
+                     "checksum": "SHA-256:c9237bfe0f62842d7187a39495baa4a7e3ab8b87c0b433614294b023cf0bc0f3",
+                     "size": "292"
                   }
                ]
             },
@@ -352,4 +352,4 @@
          ]
       }
    ]
-}
+}