functions: Use a more specific stem in getLatestSnapshot

The main component was not being captured in the previous snapshots because
"debian-buster-" would also select -nonfree and -contrib and miss the main repo.

Change-Id: Idcbd0141cef00d4ba21e1eb5080f60164ba2919b
diff --git a/vars/functions.groovy b/vars/functions.groovy
index 9ed7e7a..cce19a2 100644
--- a/vars/functions.groovy
+++ b/vars/functions.groovy
@@ -15,7 +15,7 @@
 String getLatestSnapshot(repository_stem) {
     def script = """
         aptly snapshot list --sort=time --raw \
-            | grep -E '^${repository_stem}-' \
+            | grep -E '^${repository_stem}-[0-9]' \
             | tail -n1
     """