CI: Remove uses of `nix_path`

[RyanGlScott]

Currently, the Nix-based CI specifies a nix_path with each usage of cachix/install-nix-action@v20, e.g.,

llvm-pretty-bc-parser/.github/workflows/nix-ci.yml

Lines 26 to 28 in 4934fb3

	- uses: cachix/install-nix-action@v20
	with:
	nix_path: nixpkgs=channel:nixos-23.05

It should be possible to remove these nix_paths nowadays, however. To quote @kquick in #347 (comment):

When this was first put together, we needed to be explicit about using a (what was at the time) version of nix supporting the newer CLI usage. That usage is now standard, so the nix-path can be removed (the actual nixpkgs used for the builds are controlled by the flake.lock and are not affected by this nix-path setting.