Use silversquirl/zig-flake

[silversquirl]

Shameless plug ofc, but my flake has a number of advantages over zig-overlay :)

• very frequent updates - each zig version should be available in the flake within half an hour of being published to ziglang.org
• fetches from community mirrors - this increases availability when ziglang.org has downtime, and also avoids DoSing ZSF
• includes a setup hook and a makePackage helper to make packaging extremely easy
• includes a dependency fetcher that doesn't need a deps.nix (just update a single depsHash instead)
• zero dependencies (no pointless flake-utils ^^')
• actively maintained - zig-overlay has open PRs that haven't even been given feedback in over a year

[silversquirl]

also, a note on versioning: it would be very nice if zls chose a specific nightly version to use (eg. zig_0_16_0_dev_9) rather than simply following the nightly alias. this means other flakes that depend on zls can have it follow their copy of zig-flake, and still run nix flake update without any fear of zls breaking due to the new version.

This would be much appreciated even if you choose to reject this PR and stick with zig-overlay! ^-^