buildFishPlugin: do not pass irrelevant arguments down

mkDerivation does not care about checkPlugins and checkFunctionDirs so let’s avoid polluting the scope.
Also remove installPath argument altogether for the same reason since it is not used at all.

pkgs/shells/fish/plugins/build-fish-plugin.nix

@@ -11,8 +11,6 @@ attrs@{
   buildPhase ? ":",
   preInstall ? "",
   postInstall ? "",
-  # name of the subdirectory in which to store the plugin
-  installPath ? lib.getName pname,
 
   checkInputs ? [],
   # plugin packages to add to the vendor paths of the test fish shell
@@ -26,7 +24,15 @@ attrs@{
   ...
 }:
 
-stdenv.mkDerivation (attrs // {
+let
+  # Do not pass attributes that are only relevant to buildFishPlugin to mkDerivation.
+  drvAttrs = builtins.removeAttrs attrs [
+    "checkPlugins"
+    "checkFunctionDirs"
+  ];
+in
+
+stdenv.mkDerivation (drvAttrs // {
   inherit name;
   inherit unpackPhase configurePhase buildPhase;