chore: use Lake remote cache in CI

.github/workflows/build-template.yml

@@ -131,7 +131,7 @@ jobs:
           else
             echo "TARGET_STAGE=stage1" >> $GITHUB_ENV
           fi
-      - name: Build
+      - name: Configure Build
         run: |
           ulimit -c unlimited  # coredumps
           [ -d build ] || mkdir build
@@ -168,6 +168,22 @@ jobs:
           fi
           # contortion to support empty OPTIONS with old macOS bash
           cmake .. --preset ${{ matrix.CMAKE_PRESET || 'release' }} -B . ${{ matrix.CMAKE_OPTIONS }} ${OPTIONS[@]+"${OPTIONS[@]}"} -DLEAN_INSTALL_PREFIX=$PWD/..
+      - name: Build Stage 0
+        run: |
+          ulimit -c unlimited  # coredumps
+          cd build
+          time make stage0 -j$NPROC
+      - name: Download Lake Cache
+        if: matrix.name == 'Linux Lake (Cached)'
+        run: |
+          cd src
+          ../build/stage0/bin/lake cache get \
+            --repo=${{ github.event.pull_request.head.repo.full_name || github.repository }}
+        continue-on-error: true
+      - name: Build Target Stage
+        run: |
+          ulimit -c unlimited  # coredumps
+          cd build
           time make $TARGET_STAGE -j$NPROC
       # Should be done as early as possible and in particular *before* "Check rebootstrap" which
       # changes the state of stage1/
@@ -187,6 +203,19 @@ jobs:
             build/stage1/**/*.c
             build/stage1/**/*.c.o*' || '' }}
           key: ${{ steps.restore-cache.outputs.cache-primary-key }}
+      - name: Upload Lake Cache
+        # Caching on cancellation created some mysterious issues perhaps related to improper build
+        # shutdown
+        if: matrix.name == 'Linux Lake' && !cancelled()
+        run: |
+          cd src
+          ../build/stage0/bin/lake build -o build/lake-mappings.jsonl
+          ../build/stage0/bin/lake cache put build/lake-mappings.jsonl \
+            --repo=${{ github.event.pull_request.head.repo.full_name || github.repository }}
+        env:
+          LAKE_CACHE_KEY: ${{ secrets.LAKE_CACHE_KEY }}
+          LAKE_ARTIFACT_ENDPOINT: ${{ secrets.LAKE_ARTIFACT_ENDPOINT }}
+          LAKE_REVISION_ENDPOINT: ${{ secrets.LAKE_REVISION_ENDPOINT }}
       - name: Install
         run: |
           make -C build/$TARGET_STAGE install

.github/workflows/ci.yml

@@ -200,8 +200,20 @@ jobs:
                 "test": true,
                 // NOTE: `test-speedcenter` currently seems to be broken on `ubuntu-latest`
                 "test-speedcenter": large && level >= 2,
-                // made explicit until it can be assumed to have propagated to PRs
-                "CMAKE_OPTIONS": "-DUSE_LAKE=ON",
+                // `-DUSE_LAKE=ON` made explicit until it can be assumed to have propagated to PRs
+                "CMAKE_OPTIONS": "-DUSE_LAKE=ON -DUSE_LAKE_CACHE=ON",
+              },
+              {
+                "name": "Linux Lake (Cached)",
+                "os": large ? "nscloud-ubuntu-22.04-amd64-8x16-with-cache" : "ubuntu-latest",
+                "enabled": true,
+                "check-rebootstrap": level >= 1,
+                "check-stage3": level >= 2,
+                "test": true,
+                "secondary": true,
+                // NOTE: `test-speedcenter` currently seems to be broken on `ubuntu-latest`
+                "test-speedcenter": large && level >= 2,
+                "CMAKE_OPTIONS": "-DUSE_LAKE=ON -DUSE_LAKE_CACHE=ON",
               },
               {
                 "name": "Linux Reldebug",