Provide TMPDIR for boot image repacking (tiann#2458)

/data/local/tmp is never writable for normal apps, why previously it
works is that Rust's temp_dir() gets path from env, and since A13,
TMPDIR is set to app's cache dir. This is not the case for A12, so it
breaks. Fix it by set TMPDIR ourselves.

manager/app/src/main/java/me/weishu/kernelsu/KernelSUApplication.kt

@@ -1,6 +1,7 @@
 package me.weishu.kernelsu
 
 import android.app.Application
+import android.system.Os
 import coil.Coil
 import coil.ImageLoader
 import me.zhanghai.android.appiconloader.coil.AppIconFetcher
@@ -30,7 +31,10 @@ class KernelSUApplication : Application() {
         if (!webroot.exists()) {
             webroot.mkdir()
         }
+
+        // Provide working env for rust's temp_dir()
+        Os.setenv("TMPDIR", cacheDir.absolutePath, true)
     }
 
 
-}
+}