ci: Hard-code XCTest deployment target to 10.15
Avoid using the macOS version running on the CI job host that happens to run the build job.
Backport: release
Edited by Brad King
GitLab update will be applied on June 28th, between 8am and 10:30am, EDT (UTC-04:00). This site may have intermittent delays during that time.
Avoid using the macOS version running on the CI job host that happens to run the build job.
Backport: release