[CI] Cache the Windows compile results with sccache

A pull request that touches one file rebuilds it, but sccache can hand
back the objects for everything else, and the GitHub Actions cache keeps
them between runs, so the second build of a branch costs about what the
first one stored and nothing more.

sccache has to wrap the compiler, and cmake only wraps it for the
generators that run the compiler themselves: the Visual Studio
generator drives MSBuild, which would drop a launcher on the floor
without a word. The cache therefore goes on the Ninja job, and
compile.sh now refuses --sccache on any other generator instead of
pretending to cache, and prints the sccache statistics next to the
existing ccache ones so the hit rate is visible in the log.

For MSVC, a cacheable object has to carry its debug information
inside, which is /Z7 rather than /Zi; sccache cannot restore the
latter, it can only restore a whole object file. Ask CMake for the
format by name through CMP0141 and CMAKE_MSVC_DEBUG_INFORMATION_FORMAT
instead of hardcoding /Zi in the flags, where it would end up next to
/Z7, and keep the format as it was for every build that does not
cache. The linker still gets /DEBUG, so the release PDBs are written
either way, which is what that flag is there for.

The policy is set before project(), since that is when it takes
effect, and behind an if(POLICY) guard because it does not exist
before CMake 3.25 - where the old defaults remain the only option.

.github/workflows/desktop-build.yml

@@ -415,6 +415,7 @@ jobs:
             qt_version: 6.11.*
             qt_modules: qtimageformats qtmultimedia qtshadertools qtwebsockets
             type: Release
+            use_sccache: 1
             variant: ' (Ninja)'
             # The same scope as the Visual Studio leg, so both halves of the A/B
             # start from the same vcpkg cache and the comparison is about the
@@ -521,6 +522,20 @@ jobs:
         with:
           arch: x64
 
+      # sccache is only useful for the Ninja job: a compiler cache has to wrap the
+      # compiler, and MSBuild is driven by cmake instead of running it, so a
+      # launcher handed to MSBuild is ignored without a word.
+      - name: "[Windows/Ninja] Set up sccache"
+        if: matrix.os == 'Windows' && matrix.cmake_generator == 'Ninja'
+        uses: mozilla-actions/sccache-action@v0.0.11
+        env:
+          SCCACHE_GHA_ENABLED: "true"
+          # A fork pull request can read the cache, but the token it runs with
+          # cannot write to the base branch's scope.
+          SCCACHE_GHA_RW_MODE: ${{ github.event.pull_request.head.repo.fork && 'READ_ONLY' || 'READ_WRITE' }}
+          # A cache that cannot be reached must not fail the build.
+          SCCACHE_IGNORE_SERVER_IO_ERROR: "1"
+
       - name: "[Windows] Install NSIS"
         if: matrix.os == 'Windows'
         shell: bash
@@ -569,8 +584,9 @@ jobs:
           MACOS_CI_KEYCHAIN_PWD: ${{ secrets.PROD_MACOS_CI_KEYCHAIN_PWD }}
           MAKE_PACKAGE: '${{ matrix.make_package }}'
           PACKAGE_SUFFIX: '${{ matrix.package_suffix }}'
-          TARGET_MACOS_VERSION: ${{ matrix.override_target }}
+          TARGET_MACOS_VERSION: '${{ matrix.override_target }}'
           USE_CCACHE: ${{ matrix.use_ccache }}
+          USE_SCCACHE: ${{ matrix.use_sccache }}
           # Only pushes to the default branch may write the shared binary cache: the
           # archives a run compiles are only as trustworthy as the code they were
           # built from, and only merged code may populate the scope that master and