Don't rerun nix-build when nothing has changed

[matthewbauer]

Currently, if you have a failing nix-build result, nix-buffer will continually try to run (when used as a find-file-hook). We should try to "mark" these failing builds once and avoid rebuilding until the inputs change.

[shlevy]

👍 one way to do this is to cache the most recent failed drv path in the cache dir and check it before running the build.

[matthewbauer]

So, can you get the drv path from a failed nix-build call? I've looked into it and it looks like the easiest way is to call nix-instantiate to get a drv then run nix-store -r to build it. The thing is that doesn't seem to setup the out-link stuff that nix-buffer uses. But, maybe we don't need that if we can just store drv paths.

[shlevy]

You can set up out-link stuff with nix-store. See the --add-root and --indirect flags.

[shlevy]

Will take this up in the flake mode