Move minimal-versions CI to dedicated job

Author: dtolnay

.github/workflows/ci.yml

@@ -74,7 +74,18 @@ jobs:
         run: cargo test
         env:
           RUSTFLAGS: -Z allow-features= ${{env.RUSTFLAGS}}
-      - run: cargo update -Z minimal-versions && cargo build
+
+  minimal:
+    name: Minimal versions
+    needs: pre_ci
+    if: needs.pre_ci.outputs.continue
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    steps:
+      - uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@nightly
+      - run: cargo generate-lockfile -Z minimal-versions
+      - run: cargo check --locked
 
   webassembly:
     name: WebAssembly