Add a `-withBlender` CLI argument for a unit test, to aid in debugging
T99438.
Run the test with `go test ./internal/worker/find_blender/ -args -withBlender`
to actually fail when the file association with `.blend` files cannot be
found.
Note that this doesn't rely on Blender being runnable, but it does rely
on _something_ being associated with .blend files.
`os.IsNotExist()` is from before `errors.Is()` existed. The latter is the
recommended approach, as it also recognised wrapped errors.
No functional changes, except for recognising more cases of "does not
exist" errors as such.
