[nix] bump nixpkgs to 22.05, spago now at 0.20.9

Now the previous `build` script works (from `shell.nix`).
2 jobs for 395-dev-ps-0.15-update in 0 seconds (queued for 77 minutes and 47 seconds)
Status Job ID Name Coverage
  Test
failed #4929
deps

failed #4930
test

 
Name Stage Failure
failed
test Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace
failed
deps Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace