gitlab-ci: avoid collisions with other projects' sccache servers

Shell runners might be running multiple projects in parallel. Avoid
conflicting with their sccache servers by using different port
selections.

.gitlab/os-macos.yml

@@ -9,6 +9,8 @@
         # 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-11.5.app/Contents/Developer"
+        # Avoid conflicting with other projects running on the same machine.
+        SCCACHE_SERVER_PORT: 4231
 
 ### Build and test
 

.gitlab/os-windows.yml

@@ -6,6 +6,8 @@
     variables:
         GIT_SUBMODULE_STRATEGY: recursive
         GIT_CLONE_PATH: "$CI_BUILDS_DIR\\cmb-ci-ext\\$CI_CONCURRENT_ID"
+        # Avoid conflicting with other projects running on the same machine.
+        SCCACHE_SERVER_PORT: 4231
 
 ### Build and test