omnixrc - needs to respect cached devshell

[srid]

It should not re-run health and other stuff when nix-direnv will be using the cached devshell. Otherwise there is perceptible delay every time we cd to the project dir.

[shivaraj-bh]

What if we have omnix-direnv like nix-direnv? omnix-direnv will provide custom direnvrc configuration to support preShell and postShell. In order to respect cached devShell we can use the same strategy as nix-direnv to cache the output of nix print-dev-env, see https://github.com/nix-community/nix-direnv/blob/7789681eb28fae8de052866f14d009f2375f9362/direnvrc#L335-L349. omnix-direnv will cache three devShells instead of just one. I haven’t completely thought this through, just wanted to put this idea out.