clangarm64: let the tests pass! (#5586)

I encountered these issues that had hitherto escaped us [when I worked
on letting the `ci-artifacts` workflow in git-sdk-arm64 also build Git
and run the test
suite](https://github.com/git-for-windows/git-sdk-arm64/pull/37) by way
of validating the `minimal-sdk` artifact.

Mind, this PR does not only adjust a test case that was previously too
fixated on x86_64. There are two real issues that this PR addresses and
that were found via the test suite:

- When the environment variable `MSYSTEM` is not yet set, it now is set
appropriately even on Windows/ARM64 (and the `PATH` is adjusted
accordingly).
- The tree traversal limit designed to avoid stack overflows needed to
be adjusted for the clangarm64 builds.

environment.c

@@ -101,9 +101,21 @@ int max_allowed_tree_depth =
 	 * tree depth; This value seems to be low enough.
 	 */
 	1280;
+#else
+#if defined(GIT_WINDOWS_NATIVE) && defined(__clang__) && defined(__aarch64__)
+	/*
+	 * Similar to Visual C, it seems that on Windows/ARM64 the clang-based
+	 * builds have a smaller stack space available. When running out of
+	 * that stack space, a `STATUS_STACK_OVERFLOW` is produced. When the
+	 * Git command was run from an MSYS2 Bash, this unfortunately results
+	 * in an exit code 127. Let's prevent that by lowering the maximal
+	 * tree depth; This value seems to be low enough.
+	 */
+	1280;
 #else
 	2048;
 #endif
+#endif
 
 #ifndef PROTECT_HFS_DEFAULT
 #define PROTECT_HFS_DEFAULT 0