Replace the CI build on Buildkite with GitHub Actions.

The GitHub Actions logs are public, just like this repository.

We don't need the Docker setup for GitHub Actions; `rustup` is enough.

.github/workflows/build.yaml

@@ -0,0 +1,24 @@
+name: build
+
+on:
+  push:
+
+jobs:
+  cargo-build:
+    name: cargo build
+    runs-on: ubuntu-latest
+    env:
+      CARGO_NET_GIT_FETCH_WITH_CLI: "true"
+      RUSTFLAGS: "-D warnings" # fail on warnings
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: install tools
+        run: |
+          rustup show
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: build
+        run: |
+          cargo build --release

rust-toolchain.toml

@@ -0,0 +1,4 @@
+[toolchain]
+channel = "1.76.0"
+profile = "default"  # see https://rust-lang.github.io/rustup/concepts/profiles.html
+components = ["rust-analyzer", "rust-src"]  # see https://rust-lang.github.io/rustup/concepts/components.html