Do not hardcode the path to genrule-setup.sh in the implementation of genrule.

I don't know why it was implemented like that in the first place.

--
MOS_MIGRATED_REVID=103934038
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/rules/genrule/GenRule.java b/src/main/java/com/google/devtools/build/lib/bazel/rules/genrule/GenRule.java
index 1b6284c..3f931e2 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/rules/genrule/GenRule.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/rules/genrule/GenRule.java
@@ -50,9 +50,6 @@
  */
 public class GenRule implements RuleConfiguredTargetFactory {
 
-  public static final String GENRULE_SETUP_CMD =
-      "source tools/genrule/genrule-setup.sh; ";
-
   private Artifact getExecutable(RuleContext ruleContext, NestedSet<Artifact> filesToBuild) {
     if (Iterables.size(filesToBuild) == 1) {
       Artifact out = Iterables.getOnlyElement(filesToBuild);
@@ -99,7 +96,9 @@
         ruleContext.attributes().get("heuristic_label_expansion", Type.BOOLEAN), false);
 
     // Adds the genrule environment setup script before the actual shell command
-    String command = GENRULE_SETUP_CMD + baseCommand;
+    String command = String.format("source %s; %s",
+        ruleContext.getPrerequisiteArtifact("$genrule_setup", Mode.HOST).getExecPath(),
+        baseCommand);
 
     command = resolveCommand(ruleContext, command, resolvedSrcs, filesToBuild);