Make the terraform harness only use the rwmutex if the plugin cache is enabled

[toastwaffle]

Description of your changes

As discussed in #239, this makes the terraform harness only take the lock if the plugin cache is enabled. If the plugin cache is not enabled, there is nothing to protect, and thus no need to take the lock.

This change also removes locking for all terraform plan operations, on the assumption that conflicts are unlikely, and can easily be retried on a subsequent reconcile if they do happen.

I have:

• [x] Run make reviewable to ensure this PR is ready for review. It returned a bunch of type errors which seemed unrelated to this change, but I ran make test which did pass and make build which did succeed.

How has this code been tested

Unit tests

[toastwaffle]

I have now tested this locally with 4 workspaces with the plugin cache disabled doing long-running applies (GKE cluster creation) in parallel (where previously they'd have gotten stuck), and 4 other workspaces with the plugin cache enabled successfully reconciling some DNS records

[toastwaffle]

@bobh66 does this qualify for an off-schedule release, or does this need to wait until the 29th?

[bobh66]

Good question - seems like a good candidate for a backport - I'll see what the release team thinks

[github-actions[bot]]

Successfully created backport PR #242 for release-0.14.