dist/tools/ci/print_toolchain_versions.sh: also support git worktrees

[mguetschow]

Contribution description

Inside git worktrees, .git is not a folder, but a regular file. The script currently explicitly checks for an existing folder. It is anyhow better to use git tooling to check that, so do this instead.

Testing procedure

$ git worktree add -d ../test
$ cd ../test
$ make print-versions

will print on master:

RIOT version information
----------------------------
              RIOT branch: unknown (not a git repository)
         RIOT commit hash: unknown (not a git repository)
         RIOT commit date: unknown (not a git repository)

With this PR, it works as expected, also if not in a git worktree

[riot-ci]

Murdock results

✔️ PASSED

7db49b1 dist/tools/ci/print_toolchain_versions.sh: also support git worktrees

Success	Failures	Total	Runtime
1	0	1	01m:17s

Artifacts

• Documentation preview

[crasbe]

The static test has a (valid) remark about your change and a very valid remark about line 57 (not your code, but still a bug). Perhaps you can address at least these two shellcheck comments?

It would be extra-kind if you also fixed the other ones while you're at it 😇