PGO update: Only skip when there is no change in runsc/profiles.

The BuildKite pipeline modifies `.buildkite/pipeline.yaml` at runtime,
causing `git status` to return a diff on that file at all times.
Therefore, the check that files were not modified always failed, as
`.buildkite/pipeline.yaml` was always seen as modified.

This CL makes the check only look at changes in `runsc/profiles`.

PiperOrigin-RevId: 774229651

Author: EtiennePerot

.buildkite/scripts/pgo/maybe-skip.sh

@@ -53,7 +53,7 @@ if [[ "$(gh pr --repo="$repo_url" list --label pgo-update --state open --json ti
 fi
 
 if ! echo "$@" | grep -qFe '--tolerate-no-profile-changes'; then
-  if [[ "$(git status --porcelain | wc -l)" == 0 ]]; then
+  if [[ "$(git status --porcelain runsc/profiles | wc -l)" == 0 ]]; then
     echo "No changes to runsc profiles; skipping." >&2
     exit 0
   fi