[nix-local-build] Recompilation avoidance ABA problem

Steps to reproduce:
1. Build a project with new-build
2. Modify a file in the project
3. Trigger a recompilation, but before GHC is done executing, C^C it
4. Revert the modified file to its old version
5. Trigger a recompilation

Expected result: Cabal recompiles

Actual result: Cabal reports that there is nothing to do.

Ostensibly, the problem is that before we start a recompile, we need to invalidate the old cached entry until the recompile finishes, so that we can handle an interrupt.

Start by reproducing the five-step sequence with nix-local-build and trace how recompilation state is cached around an interrupted GHC run. Done means that after cancelling recompilation and reverting the file, the next build detects the change and recompiles instead of reporting nothing to do.

Written by the indexing model from the issue text.