This PR is 2 fixes for the flaky TestTaskRunner_TaskEnv_Chroot test.
And also the TestTaskRunner_Download_ChrootExec test.
- Use TinyChroot to stop copying gigabytes of junk, which causes GHA
to fail to create the environment in time.
- Pre-create cgroups on V2 systems. Normally the cgroup directory is
managed by the cpuset manager, but that is not active in taskrunner tests,
so create it by hand in the test framework.
On macOS, `os.TempDir` returns a symlinked path under `/var` which is
outside of the directories shared into the VM used for Docker, and
that fails tests using Docker that need that mount. If we expand the
symlink to get the real path in `/private`, we're in the shared
folders and can safely mount them.
