gitlab-ci: get the sccache server port from the runner

Our CI setup has per-project runner setups, so they are now providing
the port configuration. The main goal is to have Rosetta caches separate
from the native caches on macOS, but removing this removes a bit of
coupling to Kitware's CI setup.

.gitlab/os-macos.yml

@@ -8,8 +8,6 @@
         # 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-26.1.app/Contents/Developer"
-        # Avoid conflicting with other projects running on the same machine.
-        SCCACHE_SERVER_PORT: 4227
 
 ### Build and test
 

.gitlab/os-windows.yml

@@ -5,8 +5,6 @@
 .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