ci: CMake: Pin Boost version on macOS

Disable MSYS2 job because there are no trivial ways to install older
Boost.

.github/workflows/cmake-ci.yml

@@ -150,7 +150,7 @@ jobs:
             - name: Install dependencies
               run: brew install --overwrite
                   ${{matrix.deps}}
-                  boost
+                  boost@1.85
                   ccache
                   libzip
                   ninja
@@ -168,6 +168,7 @@ jobs:
                   -DCMAKE_C_COMPILER_LAUNCHER=ccache
                   -DCMAKE_CXX_COMPILER_LAUNCHER=ccache
                   ${{matrix.options}}
+                  -DBOOST_ROOT=$(brew --prefix [email protected])
                   -DENABLE_TESTS=ON
             - name: Restore cache
               uses: actions/cache/restore@v4
@@ -195,6 +196,8 @@ jobs:
                   retention-days: 7
 
     test-msys2:
+        # MSYS2 is rolling release, only latest Boost is available.
+        if: false
         runs-on: windows-latest
         defaults:
             run: