ci: Add a Github Action for Rust lints

I'd like to move `cargo fmt` checking out of Jenkins; among
other things, GH actions are *much* faster and nicer for this.
This leaves Jenkins to be more heavyweight testing.

.github/workflows/rust-lints.yml

@@ -0,0 +1,27 @@
+---
+name: Rust
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+env:
+  CARGO_TERM_COLOR: always
+  ACTIONS_LINTS_TOOLCHAIN: 1.50.0
+
+jobs:
+  linting:
+    name: "cargo fmt"
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+      - name: Install toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: ${{ env['ACTIONS_LINTS_TOOLCHAIN']  }}
+          default: true
+          components: rustfmt
+      - name: cargo fmt (check)
+        run: cargo fmt -- --check -l