Fix version when using source tarballs (#7164)

In tarballs there is not git information and `git` would check every
parent folder potentially returning unrelated information.

Verify that `setup.py` is run from a git checkout and bail out
otherwise.
The existence of the `.git` folder should be a good enough indicator for
that.


# New contributor declaration
- [x] I am not making a trivial change, such as fixing a typo in a
comment.

- [x] I have written a PR description following these
  [rules](https://cbea.ms/git-commit/#why-not-how).

- [x] I have run `pre-commit run --from-ref origin/main --to-ref HEAD`.

- Select one of the following.
- [x] This PR does not need a test because it changes the setup.py file
only

- Select one of the following.
  - [x] I have not added any `lit` tests.

---------

Co-authored-by: peterbell10 <[email protected]>

Author: Flamefire

setup.py

@@ -756,6 +756,8 @@ def get_git_branch():
 
 
 def get_git_version_suffix():
+    if not (Path(__file__).parent / ".git").is_dir():
+        return ""  # Not a git checkout
     branch = get_git_branch()
     if branch.startswith("release"):
         return ""