perf(ci): skip rust-cache when S3 sccache is configured

The Swatinem/rust-cache was caching ~10GB and taking 3+ minutes to
extract on runs-on self-hosted runners. Since these runners already
have S3-backed sccache configured via runs-on/action, the rust-cache
was redundant.

This change automatically detects when S3 sccache is configured by
checking for the SCCACHE_BUCKET environment variable (set by
runs-on/action when sccache: s3 is used). When detected, rust-cache
is skipped. Jobs on ubuntu-latest without S3 sccache still benefit
from rust-cache.

Additional optimizations:
- Remove redundant VS Build Tools install on Windows (already pre-installed
  in windows22-full-x64 image) - saves ~4 minutes
- Add --threads $(nproc) to grcov for explicit max parallelization

Expected improvement: ~3-4 minutes faster setup-rust on runs-on runners,
~4 minutes faster Windows job.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>
Signed-off-by: Joe Isaacs <[email protected]>

Author: joseph-isaacs

.github/workflows/ci.yml

@@ -328,6 +328,7 @@ jobs:
         run: |
           rustup component add llvm-tools-preview
           grcov . --binary-path target/debug/ -s . -t lcov --llvm --ignore-not-existing \
+            --threads $(nproc) \
             --ignore '../*' --ignore '/*' --ignore 'fuzz/*' --ignore 'bench-vortex/*' \
             --ignore 'home/*' --ignore 'xtask/*' --ignore 'target/*' --ignore 'vortex-error/*' \
             --ignore 'vortex-python/*' --ignore 'vortex-jni/*' --ignore 'vortex-flatbuffers/*' \
@@ -429,11 +430,8 @@ jobs:
         with:
           sccache: s3
       - uses: actions/checkout@v5
-      - name: Install Visual Studio Build Tools (Windows)
-        if: matrix.os == 'windows-x64'
-        shell: bash
-        run: |
-          choco install visualstudio2022buildtools --package-parameters "--add Microsoft.VisualStudio.Workload.VCTools --includeRecommended --passive" -y
+      # VS Build Tools already pre-installed on windows22-full-x64 image
+      # See: https://github.com/actions/runner-images/blob/main/images/windows/Windows2022-Readme.md
       - name: Setup Python (Windows)
         if: matrix.os == 'windows-x64'
         uses: actions/setup-python@v5