Make strings not iterable

Fixes #5830

RELNOTES: Flip flag --incompatible_string_is_not_iterable (https://github.com/bazelbuild/bazel/issues/5830)
PiperOrigin-RevId: 225981394
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java
index ac3fa3a..d58fd2c 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemantics.java
@@ -230,7 +230,7 @@
           .incompatibleRemoveNativeHttpArchive(true)
           .incompatibleRemoveNativeMavenJar(false)
           .incompatibleStricArgumentOrdering(false)
-          .incompatibleStringIsNotIterable(false)
+          .incompatibleStringIsNotIterable(true)
           .internalSkylarkFlagTestCanary(false)
           .build();