When running tests on CI, I hit a race condition which caused the

ExtensionManagerTests::test_install_media_concurrent_threads test to

fail.
The version check in install_extension_media() happened only before

acquiring the lock file. If one thread completed the install and released

the lock before another thread attempted to acquire it, the second thread

would successfully lock and redundantly reinstall, since its local

old_version was already stale.
This change makes us re-check the installed version after acquiring the

lock to skip the install if another thread/process already completed it.

Ran unit tests.