Auto label PRs that may require extra checks

We would like to automatically run benchcomp if the PR touches some
files. Use an auto label action for that.

.github/labeler.yml

@@ -0,0 +1,14 @@
+# Copyright Kani Contributors
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+#
+# Configuration for auto-labeling PRs
+#
+# Note that we enable dot, so "**" matches all files in a folder
+
+Z-BenchCI:
+  - kani-compiler/**
+  - rust-toolchain.toml
+  - kani-dependencies
+  - kani-driver/src/call-*
+  - Cargo.lock
+

.github/workflows/labeler.yml

@@ -0,0 +1,30 @@
+# Copyright Kani Contributors
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+#
+# Auto label PRs based on the files that were changed
+#
+# This PR runs on `pull_request_target` because it needs extra write permission.
+#
+# Thus, we keep this workflow minimal, and the only action used here is from a
+# verified publisher.
+#
+# See <https://github.com/actions/labeler/issues/121> for more details.
+
+name: Auto Label
+on: pull_request_target
+
+jobs:
+  auto-label:
+    permissions:
+      contents: read
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout Kani
+      uses: actions/checkout@v3
+
+    - name: Label PR
+      uses: actions/labeler@v4
+      with:
+        dot: true
+

.github/workflows/toolchain-upgrade.yml

@@ -74,7 +74,6 @@ jobs:
           branch: toolchain-${{ env.next_toolchain_date }}
           delete-branch: true
           title: 'Automatic toolchain upgrade to nightly-${{ env.next_toolchain_date }}'
-          labels: Z-BenchCI
           body: >
             Update Rust toolchain from nightly-${{ env.current_toolchain_date }} to
             nightly-${{ env.next_toolchain_date }} without any other source changes.