[nix] migrate to flake.nix

This simplifies the multitude of nix files that we have into 1 single flake.
2 jobs for dev-spago-next-with-flake in 2 minutes and 58 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Compile
failed #12904
compile

00:02:58

 
  Test
skipped #12905
test
 
Name Stage Failure
failed
compile Compile
No URL provided, cache will not be downloaded from shared cache server. Instead a local version of cache will be extracted. 
WARNING: node_modules/.bin/node-gyp-build: chmod node_modules/.bin/node-gyp-build: no such file or directory (suppressing repeats)
Successfully extracted cache
Executing "step_script" stage of the job script
Using docker image sha256:9d005781c00d336ea8057577f22ddbd2f408d1b021fd02286d0cbe93356d1055 for nixos/nix:latest with digest nixos/nix@sha256:d832e9b78fe119bdbe86549ab53557b27c837f3f02d1addc885ba294d143769b ...
$ nix run .#fix-bun
error: experimental Nix feature 'nix-command' is disabled; add '--extra-experimental-features nix-command' to enable it
Cleaning up file based variables
ERROR: Job failed: exit code 1