trivial-builders: merge passAsFile with env in runCommand'

Previously it was awkward to use the runCommand-variants with
passAsFile as a double definition of passAsFile would potentially
break runCommand: passAsFile would overwrite the previous definition,
defeating the purpose of setting it in runCommand in the first place.
This is now fixed by concatenating the [ "buildCommand" ] list with
one the one from env, if present.

Adjust buildEnv where passAsFile = null; was passed in some cases,
breaking evaluation since it'd evaluate to [ "buildCommand" ] ++ null.

pkgs/build-support/buildenv/default.nix

@@ -74,7 +74,7 @@ runCommand name
     preferLocalBuild = true;
     allowSubstitutes = false;
     # XXX: The size is somewhat arbitrary
-    passAsFile = if builtins.stringLength pkgs >= 128*1024 then [ "pkgs" ] else null;
+    passAsFile = if builtins.stringLength pkgs >= 128*1024 then [ "pkgs" ] else [ ];
   }
   ''
     ${buildPackages.perl}/bin/perl -w ${builder}

pkgs/build-support/trivial-builders.nix

@@ -6,13 +6,14 @@ let
     stdenv.mkDerivation ({
       name = lib.strings.sanitizeDerivationName name;
       inherit buildCommand;
-      passAsFile = [ "buildCommand" ];
+      passAsFile = [ "buildCommand" ]
+        ++ (env.passAsFile or []);
     }
     // (lib.optionalAttrs runLocal {
           preferLocalBuild = true;
           allowSubstitutes = false;
        })
-    // env);
+    // builtins.removeAttrs env [ "passAsFile" ]);
 
 in