.github: add action for updating web-client-prebuilt module

Updates tailscale/corp#14335

Signed-off-by: Will Norris <will@tailscale.com>

.github/workflows/update-webclient-prebuilt.yml

@@ -0,0 +1,51 @@
+name: update-webclient-prebuilt
+
+on:
+  # manually triggered
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-$${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  update-webclient-prebuilt:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v4
+
+      - name: Run go get
+        run: |
+          GOPROXY=direct ./tool/go get github.com/tailscale/web-client-prebuilt
+          ./tool/go mod tidy
+
+      - name: Get access token
+        uses: tibdex/github-app-token@b62528385c34dbc9f38e5f4225ac829252d1ea92 # v1.8.0
+        id: generate-token
+        with:
+          # TODO(will): this should use the code updater app rather than licensing.
+          # It has the same permissions, so not a big deal, but still.
+          app_id: ${{ secrets.LICENSING_APP_ID }}
+          installation_id: ${{ secrets.LICENSING_APP_INSTALLATION_ID }}
+          private_key: ${{ secrets.LICENSING_APP_PRIVATE_KEY }}
+
+      - name: Send pull request
+        id: pull-request
+        uses: peter-evans/create-pull-request@284f54f989303d2699d373481a0cfa13ad5a6666 #v5.0.1
+        with:
+          token: ${{ steps.generate-token.outputs.token }}
+          author: OSS Updater <noreply+oss-updater@tailscale.com>
+          committer: OSS Updater <noreply+oss-updater@tailscale.com>
+          branch: actions/update-webclient-prebuilt
+          commit-message: "go.mod: update web-client-prebuilt module"
+          title: "go.mod: update web-client-prebuilt module"
+          body: Triggered by ${{ github.repository }}@${{ github.sha }}
+          signoff: true
+          delete-branch: true
+          reviewers: ${{ github.triggering_actor }}
+
+      - name: Summary
+        if: ${{ steps.pull-request.outputs.pull-request-number }}
+        run: echo "${{ steps.pull-request.outputs.pull-request-operation}} ${{ steps.pull-request.outputs.pull-request-url }}" >> $GITHUB_STEP_SUMMARY