Flip Skymeld for Bazel@HEAD.

Bazel@HEAD + Downstream: https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/3355#_

The above run didn't perform worse than the latest [automated daily run](https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/3353#_)

#14057

PiperOrigin-RevId: 570950844
Change-Id: I0c38839cb2b9e979265bbe3bff903a54162da4f4

src/main/java/com/google/devtools/build/lib/buildtool/BuildRequestOptions.java

@@ -375,7 +375,7 @@ public String getSymlinkPrefix(String productName) {
    */
   @Option(
       name = "experimental_merged_skyframe_analysis_execution",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
       metadataTags = OptionMetadataTag.EXPERIMENTAL,
       effectTags = {OptionEffectTag.LOADING_AND_ANALYSIS, OptionEffectTag.EXECUTION},