Reduce number of GitHub Actions CI builds

By reducing the number of CI jobs for GitHub actions, it should be
possible to get a faster overview over the status of all CI jobs. While
this does increase the total build time of GitHub Actions by reducing
parallelization, it should still finish within the SourceHut CI times.

.github/workflows/ci.yml

@@ -9,30 +9,19 @@ jobs:
   build:
     strategy:
       matrix:
-        rust_version: [stable, 1.43.1]
         os: [windows-latest, macos-latest]
 
     runs-on: ${{ matrix.os }}
 
     steps:
       - uses: actions/checkout@v2
-      - name: Rustup
+      - name: Stable
-        run: rustup default ${{ matrix.rust_version }}
-      - name: Test
         run: cargo test
-
+      - name: Clippy
-  clippy:
+        run: |
-    strategy:
+          rustup component add clippy
-      matrix:
+          cargo clippy --all-targets
-        os: [windows-latest, macos-latest]
+      - name: Oldstable
-
+        run: |
-    runs-on: ${{ matrix.os }}
+          rustup default 1.43.1
-
+          cargo test
-    steps:
-      - uses: actions/checkout@v2
-      - name: Rustup
-        run: rustup default ${{ matrix.rust_version }}
-      - name: Install Clippy
-        run: rustup component add clippy
-      - name: Lint
-        run: cargo clippy --all-targets