add nix flake build

[jakeisnt]

• As per Add Nix flake support glif#370
• Assuming the same Rust toolchain as the glif configuration; copied that file over.

[ctrlcctrlv]

Is this OK to merge? Looks fine to me.

[jakeisnt]

Yes and no

The dev shell works perfectly and allows contributors to contribute without installing anything - i. e. 'cargo run' in 'nix shell' works perfectly

Building the package within Nix, though, doesn't work - unfortunately rust-skia uses network access to fetch a pre-compiled Skia version, and to ensure reproducibility, nix disallows network access when building a package. Without fixing this, the 'metadata' build cant be included in the other flake, because it doesn't successfully install skia, and this causes the rest of the build to fail

To solve this, an env variable needs to be set for rust-skia and a skia build has to be 'sideloaded' in; issue here, and an example package with this build can be found in nixpkgs here. I thought this would be a quick fix but ran out of time to get it in today; unfortunately I'm not sure when I'll have time to take another crack at it.

The other flake has the same problem - as of now, the dev shell works perfectly but the nix build fails.

[ctrlcctrlv]

@jakeisnt You can disable that you know. I've had to do it before. Set envvar SKIA_LIBRARY_SEARCH_PATH and/or SKIA_SOURCE_DIR. I had to do this back when @MatthewBlanchard was patching Skia to test his changes to boolean operations.

You can also pass FORCE_SKIA_BUILD to force it to build Skia which is a git submodule of skia-sys crate.