fix: install manpages during Nix installation

[hauleth]

No description provided.

[bryango]

This is no longer necessary as the man page installation has been upstreamed (by me haha), so one can simply update flake.lock and the man pages should get installed automatically.

[hauleth]

Yeah, but upstream fix will not fix cases, when I want to use current development tree, as for example when I want to use git branchless submit with GitHub PRs.

[bryango]

The flake.nix here inherits nixpkgs's packaging functions (including postInstall), but builds on top of the current git master:

git-branchless/flake.nix

Lines 18 to 26 in 3faa074

	# reuse the definition from nixpkgs git-branchless
	git-branchless = prev.git-branchless.overrideAttrs ({ meta, ... }: {
	name = "git-branchless";
	src = self;
	cargoDeps = final.rustPlatform.importCargoLock {
	lockFile = ./Cargo.lock;
	};

So we should get the best of both worlds. I specifically did this in #1246 to achieve the goal. It should really just work. Otherwise there might be a bug...

[arxanas]

Is this PR still necessary given the above? I don't use Nix, so I can't tell myself.