sccache: use a different port for CMake builds

This avoids server reuse between different projects on the same machine.

.gitlab/os-macos.yml

@@ -8,6 +8,8 @@
         # TODO: Factor this out so that each job selects the Xcode version to
         # use so that different versions can be tested in a single pipeline.
         DEVELOPER_DIR: "/Applications/Xcode-11.7.app/Contents/Developer"
+        # Avoid conflicting with other projects running on the same machine.
+        SCCACHE_SERVER_PORT: 4227
 
 ### Build and test
 

.gitlab/os-windows.yml

@@ -5,6 +5,8 @@
 .windows:
     variables:
         GIT_CLONE_PATH: "$CI_BUILDS_DIR\\cmake ci ext\\$CI_CONCURRENT_ID"
+        # Avoid conflicting with other projects running on the same machine.
+        SCCACHE_SERVER_PORT: 4227
 
 ### Build and test