Make Starlark actions that have an unused inputs list unshareable.

This is useful because shared actions with state are not nice, so let's
minimize the number of their occurrences.

RELNOTES: None.
PiperOrigin-RevId: 255565510

src/main/java/com/google/devtools/build/lib/analysis/actions/StarlarkAction.java

@@ -109,6 +109,11 @@ public Optional<Artifact> getUnusedInputsList() {
     return unusedInputsList;
   }
 
+  @Override
+  public boolean isShareable() {
+    return !unusedInputsList.isPresent();
+  }
+
   @Override
   public boolean discoversInputs() {
     return unusedInputsList.isPresent();

src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java

@@ -710,6 +710,7 @@ public void testCreateStarlarkActionArgumentsWithUnusedInputsList() throws Excep
     assertThat(action.getUnusedInputsList()).isPresent();
     assertThat(action.getUnusedInputsList().get().getFilename()).isEqualTo("a.txt");
     assertThat(action.discoversInputs()).isTrue();
+    assertThat(action.isShareable()).isFalse();
   }
 
   @Test