fix(ensure_rustc_commit): resolve repo root before running rustc -vV

If this script is sourced from a working directory outside the repo
tree, rustup won't find rust-toolchain.toml and may select whatever
default toolchain happens to be active, giving us the wrong commit
hash. We now derive the repo root from BASH_SOURCE (two levels up from
the script's own directory) and cd there before invoking rustc, so the
toolchain selection stays correct regardless of the caller's CWD.

Author: cds-amal

tests/ui/ensure_rustc_commit.sh

@@ -13,7 +13,11 @@ set -u
 
 # Derive the rustc commit from the active toolchain; rust-toolchain.toml
 # selects the nightly, so this stays in sync automatically.
-RUSTC_COMMIT=$(rustc -vV | grep 'commit-hash' | cut -d' ' -f2)
+# Run from the repo root so rustup picks up rust-toolchain.toml even when
+# the caller's CWD is outside the repository.
+_SCRIPT_DIR=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" &>/dev/null && pwd)
+_REPO_ROOT=$(cd -- "$_SCRIPT_DIR/../.." &>/dev/null && pwd)
+RUSTC_COMMIT=$(cd "$_REPO_ROOT" && rustc -vV | grep 'commit-hash' | cut -d' ' -f2)
 if [ -z "$RUSTC_COMMIT" ]; then
     echo "Error: Could not determine rustc commit-hash from 'rustc -vV'"
     exit 1