Fixes genrules to work correctly with bazel run when the genrule's ex…

…ecutable

attribute is set to true.
Fixes #918

RELNOTES: Genrules correctly work when used with bazel run.

--
MOS_MIGRATED_REVID=114803736

src/main/java/com/google/devtools/build/lib/bazel/rules/genrule/BazelGenRuleRule.java

@@ -179,7 +179,10 @@ are always generated into a predictable location (available via <code>$(@D)</cod
         </p>
         <p>Declaring data dependencies for the generated executable is not supported.</p>
         <!-- #END_BLAZE_RULE.ATTRIBUTE --> */
-        .add(attr("executable", BOOLEAN).value(false))
+        .add(attr("executable", BOOLEAN).value(false).nonconfigurable(
+            "Used in computed default for $is_executable, which is itself non-configurable (and "
+            + " thus expects its dependencies to be non-configurable), because $is_executable"
+            + " is called from RunCommand.isExecutable, which has no configuration context"))
 
         // TODO(bazel-team): stamping doesn't seem to work. Fix it or remove attribute.
         .add(attr("stamp", BOOLEAN).value(false))