Add -pkvm_experimental to the microdroid_gki_android14-6.1 artifacts

Current name suggests that android14-6.1 GKI can be run as guest, which
is not true. Resolve this confusion by explicitly specifying
-pkvm_experimental to the name, which points out that this image was
built from the android14-6.1-pkvm_experimental branch.

Bug: 329383978
Test: AVF presubmit
Test: vm run-microdroid --gki android14-6.1-pkvm_experimental
Change-Id: I0b93e0aacb9dde58c9a3c696d5a073e1800ec325
diff --git a/apex/sign_virt_apex.py b/apex/sign_virt_apex.py
index 7c59b54..55fe6d2 100644
--- a/apex/sign_virt_apex.py
+++ b/apex/sign_virt_apex.py
@@ -498,7 +498,7 @@
     RunCommand(args, cmd)
 
 
-gki_versions = ['android14-6.1']
+gki_versions = ['android14-6.1-pkvm_experimental']
 
 # dict of (key, file) for re-sign/verification. keys are un-versioned for readability.
 virt_apex_non_gki_files = {