Replace BuildConfiguration.Fragment#collectExecutables() with a #getShellExecutable() method.

That's all it was used for anyway.

--
MOS_MIGRATED_REVID=133824769
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/CommandHelper.java b/src/main/java/com/google/devtools/build/lib/analysis/CommandHelper.java
index 1890486..eb4208a 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/CommandHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/CommandHelper.java
@@ -306,6 +306,6 @@
   private PathFragment shellPath(Map<String, String> executionInfo) {
     // Use vanilla /bin/bash for actions running on mac machines.
     return executionInfo.containsKey("requires-darwin")
-        ? new PathFragment("/bin/bash") : ruleContext.getConfiguration().getShExecutable();
+        ? new PathFragment("/bin/bash") : ruleContext.getConfiguration().getShellExecutable();
   }
 }