Update TransitionFactory.isTool to handle legacy host transitions.

PiperOrigin-RevId: 279766978
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/config/transitions/TransitionFactory.java b/src/main/java/com/google/devtools/build/lib/analysis/config/transitions/TransitionFactory.java
index e799ef3..241b795 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/config/transitions/TransitionFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/config/transitions/TransitionFactory.java
@@ -46,6 +46,11 @@
    * part of the tooling rather than a dependency of the original target.
    */
   default boolean isTool() {
+    if (isHost()) {
+      // Every host dependency is also a tool dependency.
+      return true;
+    }
+
     return false;
   }