pkgs: comma: rename 'COMMA_{,NIX}PKGS_FLAKE'

This aligns with the rust implementation of this tool [1].

[1]: 17a4f33849

home/comma/default.nix

@@ -23,7 +23,7 @@ in
     ];
 
     home.sessionVariables = {
-      COMMA_PKGS_FLAKE = cfg.pkgsFlake;
+      COMMA_NIXPKGS_FLAKE = cfg.pkgsFlake;
     };
   };
 }

pkgs/comma/comma

@@ -30,4 +30,4 @@ if [ -z "$PROGRAM" ]; then
     exit 1
 fi
 
-nix shell "${COMMA_PKGS_FLAKE:-nixpkgs}#$PROGRAM" -c "$@"
+nix shell "${COMMA_NIXPKGS_FLAKE:-nixpkgs}#$PROGRAM" -c "$@"