Add toString implementation for ExecutionPolicy.

`ExecutionPolicy` uses the default `toString` implementation making it useless
for debugging. Add an implementation producing more readable output.

PiperOrigin-RevId: 339552679

src/main/java/com/google/devtools/build/lib/exec/ExecutionPolicy.java

@@ -52,4 +52,9 @@ public boolean canRunLocallyOnly() {
   public boolean canRunLocally() {
     return locality != Locality.REMOTE_ONLY;
   }
+
+  @Override
+  public String toString() {
+    return locality.toString();
+  }
 }