runCommand: set meta.position from the arguments

Currently all packages defined by a call to runCommand have their
meta.position pointing to trivial-builders.nix.

This change makes uses of runCommand that supply at least one attribute
in the second argument point to the right file.

pkgs/build-support/trivial-builders/default.nix

@@ -87,6 +87,10 @@ rec {
       inherit buildCommand name;
       passAsFile = [ "buildCommand" ]
         ++ (derivationArgs.passAsFile or []);
+      pos = let args = builtins.attrNames derivationArgs; in
+        if builtins.length args > 0
+        then builtins.unsafeGetAttrPos (builtins.head args) derivationArgs
+        else null;
     }
     // (lib.optionalAttrs runLocal {
           preferLocalBuild = true;