sqlx depends on CARGO environment variable

[nyarly]

I'm unsure whether there's a sensible way to fix this, but sqlx macros use $CARGO to call cargo metadata.

I'm considering pulling in cargo and then overriding in an environment variable? That seems kind of ridiculous though.

[nyarly]

I've worked around this for now with:

            wag-the-pig = cargoNix.rootCrate.build.overrideAttrs (previousAttrs: {
              buildPhase = ''
                export CARGO=${pkgs.cargo}/bin/cargo

              ''
              + previousAttrs.buildPhase;
            })

Maybe sqlx should accept a CARGO_METADATA environment variable, but it would need to be provided by crate2nix or buildRustCrate