chore: Fallback to force clone on any error attempting repo reuse

[lukemassa]

what

Make it so that, if we attempt to reuse the clone directory but run into some error, fall back to forcing the clone.

why

There are some subtle bugs when it comes to reusing the clone directory (example #6009, #6000). When I wrote the code (#5895) I was hoping it would be "good enough", but now I'm thinking that if something does go wrong, it's probably best to fallback to the known good path, and log an error.

tests

I added a unit test, and also ran this code on a branch before #6010 and watched it bypass the problem:

{"level":"info","ts":"2025-12-06T14:04:58.026-0500","caller":"events/working_dir.go:141","msg":"repo was already cloned but branch is not at correct commit, updating to \"a81b4d45a8d7a9b5920bf7456de0bbfd7d04c1db\"","json":{"repo":"lukemassa/atlantis-test","pull":"38"}}
{"level":"error","ts":"2025-12-06T14:04:59.278-0500","caller":"events/working_dir.go:111","msg":"An error occurred attempting to reuse the clone dir, falling back to forced clone. This is likely a bug please report: post-merge verification failed: HEAD^2 != a81b4d45a8d7a9b5920bf7456de0bbfd7d04c1db"
...
{"level":"info","ts":"2025-12-06T14:04:59.287-0500","caller":"events/working_dir.go:346","msg":"creating dir '/Users/lmassa/.atlantis/repos/lukemassa/atlantis-test/38/default'","json":{"repo":"lukemassa/atlantis-test","pull":"38"}}

references

Addresses an issue in: #5895
Would have prevented: #6000, #6009

[lukemassa]

I didn't introduce this variable I just moved it.