Make cargo doc check a separate job in CI

Currently, our CI script is basically completely serialized: all the
steps that we perform on a given toolchain run after each other. That
has a negative impact on turn around time, because the checks that fail
most frequently (due to small nits pointed out by clippy or a format
violation detected by rustfmt) are run pretty much last. Essentially, we
don't fail fast.
As a first step towards improving the situation, this change makes the
cargo-doc check a separate job that can run in parallel to everything
else. It means that we now only check docs on the most recent stable
toolchain, but that's entirely sufficient.

Signed-off-by: Daniel Müller <deso@posteo.net>

Author: d-e-s-o

.github/workflows/rust.yml

@@ -63,8 +63,18 @@ jobs:
     - if: ${{ matrix.rust != 'nightly' }}
       name: Run clippy
       run: cargo clippy --tests -- -D warnings
-    - if: ${{ matrix.rust != 'nightly' }}
-      name: Check documentation
-      env:
-        RUSTDOCFLAGS: '-D warnings'
-      run: cargo doc --no-deps
+  cargo-doc:
+    name: Check documentation
+    runs-on: ubuntu-latest
+    env:
+      RUSTDOCFLAGS: '-D warnings'
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install deps
+        run: sudo apt-get install -y libelf-dev
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+      - run: cargo doc --no-deps