fix(scripts/githooks): clear all repo-local Git env vars in hooks (#24138)

## Problem

In linked worktrees, Git hooks inherit multiple repo-local environment
variables: `GIT_DIR`, `GIT_COMMON_DIR`, `GIT_INDEX_FILE`, and others.
The
pre-commit and pre-push hooks only unset `GIT_DIR`, leaving the rest in
place.

When `make pre-commit` runs `go build`, Go tries to stamp VCS info by
shelling
out to `git`. With the leftover partial Git environment, `git` exits 128
and
the build fails:

```
error obtaining VCS status: exit status 128
    Use -buildvcs=false to disable VCS stamping.
```

This only happens inside hooks in a linked worktree — running `make
pre-commit`
directly from the terminal works fine because the repo-local vars are
not set.

## Fix

Replace the bare `unset GIT_DIR` in both hooks with a loop that clears
every
variable reported by `git rev-parse --local-env-vars`:

```sh
while IFS= read -r var; do
    unset "$var"
done < <(git rev-parse --local-env-vars)
```

This covers all 15 repo-local variables Git may inject (`GIT_DIR`,
`GIT_COMMON_DIR`, `GIT_INDEX_FILE`, `GIT_OBJECT_DIRECTORY`, etc.) and is
forward-compatible — if Git adds new local vars in the future, the loop
picks
them up automatically.

scripts/githooks/pre-commit

@@ -14,7 +14,15 @@
 set -euo pipefail
 
 cd "$(git rev-parse --show-toplevel)"
-unset GIT_DIR
+
+# Unset all repo-local Git env vars, not just GIT_DIR. In linked
+# worktrees the hook inherits variables like GIT_COMMON_DIR and
+# GIT_INDEX_FILE that confuse child processes (notably Go's VCS
+# stamping, which shells out to git and gets exit status 128).
+# Process substitution (not a pipe) so unset runs in the current shell.
+while IFS= read -r var; do
+	unset "$var"
+done < <(git rev-parse --local-env-vars)
 
 # In linked worktrees, set worktree-scoped hooksPath to override shared config.
 if [[ "$(git rev-parse --git-dir)" != "$(git rev-parse --git-common-dir)" ]]; then

scripts/githooks/pre-push

@@ -30,7 +30,15 @@ ALLOWLIST=(
 )
 
 cd "$(git rev-parse --show-toplevel)"
-unset GIT_DIR
+
+# Unset all repo-local Git env vars, not just GIT_DIR. In linked
+# worktrees the hook inherits variables like GIT_COMMON_DIR and
+# GIT_INDEX_FILE that confuse child processes (notably Go's VCS
+# stamping, which shells out to git and gets exit status 128).
+# Process substitution (not a pipe) so unset runs in the current shell.
+while IFS= read -r var; do
+	unset "$var"
+done < <(git rev-parse --local-env-vars)
 
 # In linked worktrees, set worktree-scoped hooksPath to override shared config.
 if [[ "$(git rev-parse --git-dir)" != "$(git rev-parse --git-common-dir)" ]]; then