git-mirrors promoted from experiment, enabled by git-mirrors-path

[pda]

This draft pull request promotes the git-mirrors functionality from experiment to standard.
It still requires git-mirrors-path to be configured, otherwise it remains disabled. This makes it pretty safe to promote from experiment status, with a few caveats below.

elastic-ci-stack-for-aws consideration

Note that elastic-ci-stack-for-aws prior to buildkite/elastic-ci-stack-for-aws#698 does provide a git-mirrors-path even when the stack chooses not to enable the experiment. That means shipping this PR could result in surprising/disruptive behaviour if it's used in an elastic-stack prior to that PR merged.

test coverage

This PR regrettably removes test coverage for git-mirrors, because that coverage was being driven externally via TEST_EXPERIMENTS:

agent/bootstrap/integration/main_test.go

Line 38 in f119ac1

if exp := os.Getenv(`TEST_EXPERIMENT`); exp != "" {

… as well as special handling in BootstrapTester:

agent/bootstrap/integration/bootstrap_tester.go

Lines 111 to 119 in f119ac1

	if experiments.IsEnabled(`git-mirrors`) {
	gitMirrorsDir, err := ioutil.TempDir("", "bootstrap-git-mirrors")
	if err != nil {
	return nil, err
	}
	bt.GitMirrorsDir = gitMirrorsDir
	bt.Env = append(bt.Env, "BUILDKITE_GIT_MIRRORS_PATH="+gitMirrorsDir)
	}

… and a special CI test entrypoint:

agent/.buildkite/steps/tests.sh

Lines 9 to 10 in f119ac1

	echo '+++ Running integration tests for git-mirrors experiment'
	TEST_EXPERIMENT=git-mirrors gotestsum --junitfile "junit-${OSTYPE}-git-mirrors.xml" -- -count=1 -failfast ./bootstrap/integration

I think this PR should add some test coverage back in, but via a dedicated test which sets up a git-mirror-path at at least traverses the happy path of the code.