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](git-for-windows/git-sdk-arm64#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.

Author: dscho