ci: add more musl compiling tests

s390x musl with `allow_failures`, because it currently does not compile (see nix-rust#1835)

Author: selfisekai

.cirrus.yml

@@ -224,14 +224,24 @@ task:
     - name: Linux powerpc
       env:
         TARGET: powerpc-unknown-linux-gnu
+    - name: Linux powerpc musl
+      env:
+        TARGET: powerpc-unknown-linux-musl
     # Cross claims to support Linux powerpc64, but it really doesn't.
     # https://github.com/rust-embedded/cross/issues/441
     - name: Linux powerpc64
       env:
         TARGET: powerpc64-unknown-linux-gnu
+    - name: Linux powerpc64 musl
+      env:
+        TARGET: powerpc64-unknown-linux-musl
     - name: Linux s390x
       env:
         TARGET: s390x-unknown-linux-gnu
+    - name: Linux s390x musl
+      env:
+        TARGET: s390x-unknown-linux-musl
+      allow_failures: true
     - name: Linux x32
       env:
         TARGET: x86_64-unknown-linux-gnux32