Mark ArtifactNestedSetKey as never shareable.

I'm working toward declaring shareability only from the key instead of having methods in each of the function, key, and value.

PiperOrigin-RevId: 416333534

src/main/java/com/google/devtools/build/lib/skyframe/ArtifactNestedSetKey.java

@@ -17,6 +17,7 @@
 import com.google.devtools.build.lib.actions.Artifact;
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.skyframe.ExecutionPhaseSkyKey;
+import com.google.devtools.build.skyframe.ShareabilityOfValue;
 import com.google.devtools.build.skyframe.SkyFunctionName;
 
 /** SkyKey for {@code NestedSet<Artifact>}. */
@@ -45,13 +46,23 @@ public NestedSet<Artifact> getSet() {
     return set;
   }
 
+  @Override
+  public ShareabilityOfValue getShareabilityOfValue() {
+    // ArtifactNestedSetValue is just a promise that data is available in memory. Not meant for
+    // cross-server sharing.
+    return ShareabilityOfValue.NEVER;
+  }
+
   @Override
   public int hashCode() {
     return node.hashCode();
   }
 
   @Override
   public boolean equals(Object that) {
+    if (this == that) {
+      return true;
+    }
     return that instanceof ArtifactNestedSetKey
         && this.node.equals(((ArtifactNestedSetKey) that).node);
   }