CI: main and merge-queue builds run on the self-hosted runner (#1847)

.github/actionlint.yaml

@@ -0,0 +1,3 @@
+self-hosted-runner:
+  labels:
+    - self-hosted-fast

.github/workflows/ci.yml

@@ -134,7 +134,12 @@ jobs:
     name: Native Linux Clippy
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ubuntu-24.04
+    # Main and merge-queue runs use the self-hosted runner (warm build cache); pull requests
+    # stay on GitHub's runners, and the runner itself refuses pull-request jobs.
+    runs-on: ${{ github.event_name == 'pull_request' && 'ubuntu-24.04' || 'self-hosted-fast' }}
+    env:
+      # Outside the checkout, so `git clean` keeps the warm build between runs.
+      CARGO_TARGET_DIR: ${{ github.event_name == 'pull_request' && 'target' || '/home/ci-runner/cargo-target/clippy' }}
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
@@ -158,9 +163,13 @@ jobs:
     name: Build tests
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ubuntu-24.04
+    # Main and merge-queue runs use the self-hosted runner (warm build cache); pull requests
+    # stay on GitHub's runners, and the runner itself refuses pull-request jobs.
+    runs-on: ${{ github.event_name == 'pull_request' && 'ubuntu-24.04' || 'self-hosted-fast' }}
     timeout-minutes: 45
     env:
+      # Outside the checkout, so `git clean` keeps the warm build between runs.
+      CARGO_TARGET_DIR: ${{ github.event_name == 'pull_request' && 'target' || '/home/ci-runner/cargo-target/build-tests' }}
       CARGO_INCREMENTAL: "0"
       RUSTC_WRAPPER: sccache
       SCCACHE_GHA_ENABLED: "true"
@@ -354,9 +363,13 @@ jobs:
     name: Doc tests
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ubuntu-24.04
+    # Main and merge-queue runs use the self-hosted runner (warm build cache); pull requests
+    # stay on GitHub's runners, and the runner itself refuses pull-request jobs.
+    runs-on: ${{ github.event_name == 'pull_request' && 'ubuntu-24.04' || 'self-hosted-fast' }}
     timeout-minutes: 45
     env:
+      # Outside the checkout, so `git clean` keeps the warm build between runs.
+      CARGO_TARGET_DIR: ${{ github.event_name == 'pull_request' && 'target' || '/home/ci-runner/cargo-target/doc-tests' }}
       CARGO_INCREMENTAL: "0"
       RUSTC_WRAPPER: sccache
       SCCACHE_GHA_ENABLED: "true"