ci: Bump nightly tag on scheduled Nightly builds (#18134)

This PR makes it so after a scheduled Nightly build we also update the
`nightly` tag to keep things in sync.

It's safe to bump the tag within this Action, as it won't trigger
another Nightly build due to GitHub's recursive Action protections:

> When you use the repository's `GITHUB_TOKEN` to perform tasks, events
triggered by the `GITHUB_TOKEN`, with the exception of
`workflow_dispatch` and `repository_dispatch`, will not create a new
workflow run. This prevents you from accidentally creating recursive
workflow runs. For example, if a workflow run pushes code using the
repository's `GITHUB_TOKEN`, a new workflow will not run even when the
repository contains a workflow configured to run when `push` events
occur.
>
> —
[source](https://docs.github.com/en/actions/security-for-github-actions/security-guides/automatic-token-authentication#using-the-github_token-in-a-workflow)

Release Notes:

- N/A

.github/workflows/release_nightly.yml

@@ -171,3 +171,28 @@ jobs:
 
       - name: Upload Zed Nightly
         run: script/upload-nightly linux-targz
+
+  update-nightly-tag:
+    name: Update nightly tag
+    if: github.repository_owner == 'zed-industries'
+    runs-on: ubuntu-latest
+    needs:
+      - bundle-mac
+      - bundle-linux-x86
+      - bundle-linux-arm
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4
+        with:
+          fetch-depth: 0
+
+      - name: Update nightly tag
+        run: |
+          if [ "$(git rev-parse nightly)" = "$(git rev-parse HEAD)" ]; then
+            echo "Nightly tag already points to current commit. Skipping tagging."
+            exit 0
+          fi
+          git config user.name github-actions
+          git config user.email github-actions@github.com
+          git tag -f nightly
+          git push origin nightly --force