ci: Convert stable job to GitHub actions workflow

.builds/stable.yml

@@ -1,13 +0,0 @@
-image: archlinux
-packages:
-  - rustup
-sources:
-  - https://github.com/ruma/ruma
-tasks:
-  - rustup: |
-      # We specify --profile minimal because we'd otherwise download docs
-      rustup toolchain install stable --profile minimal
-      rustup default stable
-  - test: |
-      cd ruma
-      cargo run -p xtask --no-default-features ci stable

.github/workflows/stable.yml

@@ -0,0 +1,25 @@
+name: Rust stable
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v2
+      - name: Install stable rust toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+      - name: Run checks
+        uses: actions-rs/cargo@v1
+        with:
+          command: run
+          args: -p xtask --no-default-features ci stable