ci: bring back GitHub Actions cache for Windows

According to all known laws of informatics, sccache should work well on
Windows. But for some reason in our environment Windows builds with the
MinGW toolchain get so slow due to sccache overheads, even though our
current sccache approach with a Redis database storing compilation
aritfacts is overall a net improvement for the remaining targets. So
let's fall back to the older cache method for Windows to continue
cutting down on CI time.

.github/workflows/ci.yml

@@ -121,8 +121,9 @@ jobs:
       # https://github.com/rust-lang/rust/blob/4c8bb79d9f565115637cc6da739f8389e79f3a29/library/std/Cargo.toml#L54-L79
       OPTIMIZED_RELEASE_BUILD_FLAGS: -Z build-std -Z build-std-features=panic-unwind
 
-      # Use sccache to speed up Rust compilation
-      RUSTC_WRAPPER: sccache
+      # Use sccache to speed up Rust compilation except for Windows GNU targets, which
+      # have great sccache overheads that make builds slow down a lot
+      RUSTC_WRAPPER: ${{ endsWith(matrix.target, '-pc-windows-gnu') && '' || 'sccache' }}
 
     # Exclude in-repo PRs from running this job
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.fork
@@ -156,17 +157,22 @@ jobs:
           components: rust-src
 
       - name: 💨 Set up sccache
+        if: ${{ !endsWith(matrix.target, '-pc-windows-gnu') }} # See comment above on sccache for Windows
         uses: mozilla-actions/sccache-action@89e9040de88b577a072e3760aaf59f585da083af # v0.0.5
 
       - name: ⚙️ Prime sccache cache backend
-        shell: bash
+        if: ${{ !endsWith(matrix.target, '-pc-windows-gnu') }}
         env:
           SCCACHE_REDIS_ENDPOINT: ${{ secrets.SCCACHE_REDIS_ENDPOINT }}
           SCCACHE_REDIS_USERNAME: ${{ secrets.SCCACHE_REDIS_USERNAME }}
           SCCACHE_REDIS_PASSWORD: ${{ secrets.SCCACHE_REDIS_PASSWORD }}
           SCCACHE_REDIS_KEY_PREFIX: ${{ secrets.SCCACHE_REDIS_KEY_PREFIX }}
         run: sccache --start-server
 
+      - name: 💨 Cache Rust artifacts (Windows)
+        if: endsWith(matrix.target, '-pc-windows-gnu')
+        uses: Swatinem/rust-cache@23bce251a8cd2ffc3c1075eaa2367cf899916d84 # v2
+
       - name: 🧰 Install nextest
         if: startsWith(matrix.target, 'x86_64-') || contains(matrix.target, '-linux-')
         uses: taiki-e/install-action@331a600f1b10a3fed8dc56f925012bede91ae51f # v2