Fix nix flake build issues

Wanted to give jetzig a try today, but could not find a working build for nixos. So this is my attempt to fix it. I noticed the following issues with flake.nix: - It always uses deps.nix from main, which breaks the build when trying to build for 0.14 - It uses fetchGit without rev, which requires impure evaluation To fix this, I made the following changes: - The build will use the deps.nix file from the source tree that is built - Only fetch jetzig from github if v14 is built. Otherwise it just builds from the current directory. I also made smaller changes to try to clean up the inputs and the output packages. With those changes, I can just run `nix shell github:jrammler/jetzig/nix-build-fixes#v14` to start a shell with jetzig. Note that builds with both zig 0.15.1 as well as master currently fail, so only the v14 build works. But this seems to be a known issue (#227). The devshell still starts, but I did not test it any further.