On macOS and Windows, our jobs use a `GIT_CLONE_PATH` that does not
contain `$CI_CONCURRENCY_ID` so that the build and test jobs always use
the same path.  Since we use shell runners for these platforms, all
concurrent jobs would use the same path and conflict.  Therefore we can
only use runners with a concurrency level of 1 for jobs in the `build`
and `test` stages.  However, jobs in the `test-ext` stage configure a
new build tree for the tests and therefore can use `$CI_CONCURRENCY_ID`
and support shell runners with higher concurrency levels.

Prepare to make that distinction in the future by pinning all jobs to
runners with a `nonconcurrent` tag.