Cache .build directory for faster runtimes

[MahdiBM]

Right now Android CI is my slowest CI aside from benchmarks.

Something like this, for example, could makes things faster:

      - name: Restore .build
        if: ${{ runner.debug != '1' }}
        id: "restore-cache"
        uses: actions/cache/restore@v4
        with:
          path: .build
          key: "${{ github.event.repository.name }}-unit-tests-${{ matrix.config.mode }}-build-${{ matrix.swift.version }}-${{ matrix.macos.version }}-${{ hashFiles('./Package.resolved') }}"
          restore-keys: "${{ github.event.repository.name }}-unit-tests-${{ matrix.config.mode }}-build-${{ matrix.swift.version }}-${{ matrix.macos.version }}-"

      - name: Build ${{ github.event.repository.name }}
        run: |
          swift build \
            --build-tests \
            --enable-code-coverage \
            --explicit-target-dependency-import-check error \
            -Xswiftc -require-explicit-sendable \
            -c ${{ matrix.config.mode }}

      - name: Cache .build
        if: steps.restore-cache.outputs.cache-hit != 'true'
        uses: actions/cache/save@v4
        with:
          path: .build
          key: "${{ github.event.repository.name }}-unit-tests-${{ matrix.config.mode }}-build-${{ matrix.swift.version }}-${{ matrix.macos.version }}-${{ hashFiles('./Package.resolved') }}"

Make sure to install zstd before calling any actions/cache so the cache action automatically uses zstd instead of gzip and becomes way faster.

See my article for full info: https://swiftonserver.com/faster-github-actions-ci-for-swift-projects/

[marcprux]

I don't think caching the .build folder will help much, since most of the time is actually spent in downloading, configuring, and starting the Android emulator. You can verify this for yourself by setting run-tests: false in the workflow parameters and seeing how much faster it is.

[MahdiBM]

@marcprux run-tests: false skips the build too?

Based on the logs, the workflow already does use caching for the toolchain.

I see this in the logs:

Build complete! (191.92s)

This build that has happened, would be around 30s I expect, with caching. So that'll be 2.5 minutes saved.
That's what the .build caching helps with.