ci: allow imgtool publishing from release branches

Allow imgtool publishing to pypi.org when versions are update on the
release branches.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

.github/workflows/imgtool.yaml

@@ -2,6 +2,7 @@ on:
   push:
     branches:
       - master
+      - v*-branch
 
 name: imgtool