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.

Expected improvement: ~3 minutes faster setup-rust on runs-on runners.

🤖 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/actions/setup-rust/action.yml

@@ -37,11 +37,15 @@ runs:
         targets: "${{inputs.targets || ''}}"
         components: "${{ inputs.components || 'clippy, rustfmt' }}"
 
+    # Skip rust-cache when S3 sccache is already configured (detected via SCCACHE_BUCKET env).
+    # The rust-cache was ~10GB and took 3+ minutes to extract, while sccache
+    # S3 backend already provides efficient compile caching.
     - name: Rust Dependency Cache
+      if: ${{ !env.SCCACHE_BUCKET }}
       uses: Swatinem/rust-cache@v2
       with:
         save-if: ${{ github.ref_name == 'develop' }}
-        shared-key: "rust-cache-${{ runner.os }}-${{ runner.environment }}"  # To allow reuse across jobs
+        shared-key: "rust-cache-${{ runner.os }}-${{ runner.environment }}"
 
     - name: Rust Compile Cache
       uses: mozilla-actions/[email protected]