ci: cache target/, disable incremental, drop redundant build

Three complementary tweaks to close the gap sccache alone can't:

- CARGO_INCREMENTAL=0: reclaims the 17 incremental-mode cache misses
  per run and prevents cargo from writing incremental fingerprints
  that defeat sccache. Incremental mode is useless in CI anyway since
  each run starts from scratch.
- actions/cache for ~/.cargo and target/: sidesteps sccache's
  structural limits (proc-macro non-cacheables, clippy-vs-rustc
  separate namespaces) by caching the whole build output keyed on
  Cargo.lock. Also caches ~/.cargo/bin so the installed sccache
  binary survives between runs.
- Drop the separate 'cargo build' step: 'cargo test --workspace'
  builds everything anyway, so the standalone build was a full
  redundant workspace compile pass.

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

.gitea/workflows/ci.yml

@@ -8,6 +8,7 @@ on:
     branches: [main]
 
 env:
+  CARGO_INCREMENTAL: "0"
   RUSTC_WRAPPER: sccache
   SCCACHE_BUCKET: sccache
   SCCACHE_ENDPOINT: http://caveman.kosherinata.internal:9000
@@ -23,6 +24,19 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Cache cargo registry and target
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/bin
+            ~/.cargo/registry/index
+            ~/.cargo/registry/cache
+            ~/.cargo/git/db
+            target
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-cargo-
+
       - name: Ensure sccache with S3 support
         env:
           RUSTC_WRAPPER: ""
@@ -39,9 +53,6 @@ jobs:
       - name: Clippy
         run: cargo clippy --workspace -- -D warnings
 
-      - name: Build
-        run: cargo build --workspace
-
       - name: Test
         run: cargo test --workspace