Fix snapshot symlinks

LOCAL_MODULE_SYMLINKS was enough to install symlinks for snapshot
binaries. However, Soong now requires LOCAL_SOONG_INSTALLED_SYMLINKS,
which results in broken symlink support in snapshots.

This fixes symlink problems, and then amends the Soong snapshot test.

Bug: 220639435
Bug: 220907049
Test: manually add symlinks and try building
Test: Soong test
Change-Id: Ia75ddf3aa642558e396f1d80736310c3c8d02ad1
diff --git a/cc/snapshot_prebuilt.go b/cc/snapshot_prebuilt.go
index 753d74c..9d40ad0 100644
--- a/cc/snapshot_prebuilt.go
+++ b/cc/snapshot_prebuilt.go
@@ -680,6 +680,9 @@
 		Input:       in,
 	})
 
+	// binary snapshots need symlinking
+	p.setSymlinkList(ctx)
+
 	return outputFile
 }