auto_update_rust_bootstrap: fix dir detection logic

Apparently I had `__name__ == "__main__"` on my mind when writing this?
:)

BUG=b:323850985
TEST=Ran ./rust_tools/auto_update_rust_bootstrap.py; it printed the
TEST=right `my_dir` value.

Change-Id: I0987cd659489eb0fd525bad0b5b7de5f77292333
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/5269401
Commit-Queue: Bob Haarman <[email protected]>
Auto-Submit: George Burgess <[email protected]>
Tested-by: George Burgess <[email protected]>
Reviewed-by: Bob Haarman <[email protected]>
1 file changed