ci: run riscv64-linux jobs if ci-riscv64-linux label is applied

This needs to be a separate workflow so that it doesn't interfere with non-CI
labeling of pull requests.

.github/workflows/ci-pr-riscv64-linux.yaml

@@ -0,0 +1,34 @@
+name: ci-pr-riscv64-linux
+on:
+  pull_request:
+    types:
+      - labeled
+      - opened
+      - reopened
+      - synchronize
+concurrency:
+  # Cancels pending runs when a PR gets updated.
+  group: ${{ github.head_ref || github.run_id }}-${{ github.actor }}
+  cancel-in-progress: true
+permissions:
+  # Sets permission policy for `GITHUB_TOKEN`
+  contents: read
+jobs:
+  riscv64-linux-debug:
+    if: contains(github.event.pull_request.labels.*.name, 'ci-riscv64-linux')
+    timeout-minutes: 420
+    runs-on: [self-hosted, Linux, riscv64]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Build and Test
+        run: sh ci/riscv64-linux-debug.sh
+  riscv64-linux-release:
+    if: contains(github.event.pull_request.labels.*.name, 'ci-riscv64-linux')
+    timeout-minutes: 420
+    runs-on: [self-hosted, Linux, riscv64]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Build and Test
+        run: sh ci/riscv64-linux-release.sh