writeTextFile: fix when executable is not a bool

(cherry picked from commit 4431bab003)

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

@@ -141,7 +141,7 @@ rec {
     runCommand name
       { inherit text executable checkPhase allowSubstitutes preferLocalBuild;
         passAsFile = [ "text" ];
-        meta = lib.optionalAttrs (executable && matches != null) {
+        meta = lib.optionalAttrs (toString executable != "" && matches != null) {
           mainProgram = lib.head matches;
         } // meta;
       }