bug: for-each-repo malfunctions in worktree due to GIT_DIR

I noticed some scripts I have that utilize `git for-each-repo` don't work if I run them from within a git worktree, but they do work fine from within other directories, including the parent git clone of those worktrees. The symptom is that the command I pass to `for-each-repo` is run as if from within the worktree each time, instead of the repos fetched from the corresponding config entry.

After a bit of sleuthing with strace, I think I identified the cause: $GIT_DIR is set in the `git for-each-repo` process, and gets passed to the child git invocations it launches.

And thus, from within that worktree, `git for-each-repo --config=foo.bar -- branch` becomes the equivalent of:

GIT_DIR=/worktree-parent/.git git -C /unrelated-repo branch

Which of course is not what the `for-each-repo` invocation would be expected to do.

Observed with git 2.51.0 from Ubuntu 25.10. I haven't tested explicitly with latest, but I took a quick look at git.c and for-each-repo.c changes between there and master and didn't see anything that looked like it would affect this behavior.

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