flake.nix: we should report a more accurate version string in --version

[colemickens]

Hello.

I'm running hx from the flake and want to be a good bug reported and include my version string. However, I know right now that the tip-flake-built hx doesn't report the correct version.

Depending on how the app is written, this can be easy (patch a constant) or hard (patch a constant and then find out that clap is ignoring the constant and reaching in the cargo metadata anyway).

[archseer]

It reads the git sha, then falls back to only using the package version here:

helix/helix-term/build.rs

Lines 5 to 15 in e1a92fd

	let git_hash = Command::new("git")
	.args(&["rev-parse", "HEAD"])
	.output()
	.ok()
	.filter(|output| output.status.success())
	.and_then(|x| String::from_utf8(x.stdout).ok());
	let version: Cow<_> = match git_hash {
	Some(git_hash) => format!("{} ({})", env!("CARGO_PKG_VERSION"), &git_hash[..8]).into(),
	None => env!("CARGO_PKG_VERSION").into(),
	};

But that will work incorrectly if building outside of the helix repository but inside a package workspace (e.g. nixpkgs or alpine's git repository) so we need to include https://github.com/tree-sitter/tree-sitter/blob/af00782dfdad02f766a76058e8631cb4edf5d894/cli/build.rs#L36-L81

We could modify build.rs so it first looks for a HELIX_GIT_HASH then tries to parse git rev-head? Then we can inject that variable during a Nix build.

[the-mikedavis]

Now with the 22.03 release:

$ nix run nixpkgs#helix -- -V
helix 22.03 (d4e45fd4)
$ nix run github:helix-editor/helix -- -V
helix 22.05-dev

🎉

(see #1875)

[the-mikedavis]

We can't include the git sha in the flake output because of how Nix builds. It's either in a bare directory or not in a git directory at all (I'm not sure which), so git rev-parse HEAD fails.