ci: add sccache with MinIO backend for build caching

All Rust compilation steps now use sccache backed by MinIO S3
at caveman.kosherinata.internal:9000. Credentials via repo secrets
SCCACHE_S3_ACCESS_KEY and SCCACHE_S3_SECRET_KEY. Cache is shared
across all bare metal runners.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -7,6 +7,15 @@ on:
   pull_request:
     branches: [main]
 
+env:
+  RUSTC_WRAPPER: sccache
+  SCCACHE_BUCKET: sccache
+  SCCACHE_ENDPOINT: http://caveman.kosherinata.internal:9000
+  SCCACHE_REGION: auto
+  SCCACHE_S3_USE_SSL: "false"
+  AWS_ACCESS_KEY_ID: ${{ secrets.SCCACHE_S3_ACCESS_KEY }}
+  AWS_SECRET_ACCESS_KEY: ${{ secrets.SCCACHE_S3_SECRET_KEY }}
+
 jobs:
   check:
     name: Format, lint, build, test
@@ -26,6 +35,9 @@ jobs:
       - name: Test
         run: cargo test --workspace
 
+      - name: Show sccache stats
+        run: sccache --show-stats
+
   srpm-cortex:
     name: Build cortex SRPM
     runs-on: fedora