Fix uninterpolated ${PR_ID} in autofix branch name

The `branch:` input to peter-evans/create-pull-request was set to
`update-collections-${PR_ID}`, but Actions inputs aren't shell-
interpolated and no `PR_ID` env var is defined — so the literal
string `${PR_ID}` was ending up in the branch name (e.g.
`update-collections-${PR_ID}`).

peter-evans/create-pull-request is designed to reuse a stable
branch: if an open PR already exists for that branch, it
force-pushes new changes onto it instead of opening a new one.
That's the desired behavior for an hourly autofix job — one
rolling PR, not a new branch each hour.

Drop the suffix entirely.

Author: kenyonj

.github/workflows/collections-renames.yml

@@ -35,5 +35,5 @@ jobs:
         with:
           commit-message: "✨ Autofixing renamed/removed collection items ✨"
           committer: "github-actions[bot] <actions@github.com>"
-          branch: "update-collections-${PR_ID}"
+          branch: update-collections
           title: "✨ Autofixing renamed/removed collection items ✨"