niri: explicitly set commit hash to "Nixpkgs"

Upstream recommends[^1] that in build environments where the Git history
is not available, packagers should manually set the commit hash. Given
that currently in Nixpkgs we don't have a great way of obtaining the hash
through the usual fetchers and update scripts, we could just set it to
"Nixpkgs" for now, to indicate to the user that their Niri binary is
built from Nixpkgs and not from source, for example.

[^1]: https://github.com/YaLTeR/niri/wiki/Packaging-niri#version-string

pkgs/by-name/ni/niri/package.nix

@@ -110,6 +110,11 @@ rustPlatform.buildRustPackage (finalAttrs: {
         "-Wl,--pop-state"
       ]
     );
+
+    # Upstream recommends setting the commit hash manually when in a
+    # build environment where the Git repository is unavailable.
+    # See https://github.com/YaLTeR/niri/wiki/Packaging-niri#version-string
+    NIRI_BUILD_COMMIT = "Nixpkgs";
   };
 
   nativeInstallCheckInputs = [ versionCheckHook ];