build: fix set_variables.sh in workflow_dispatch case (#76)

Author: thebaptiste

.github/workflows/set_variables.sh

@@ -20,6 +20,13 @@ case "${GITHUB_EVENT_NAME}" in
         else
             OS_VERSION=${PAYLOAD_OS}
         fi;;
+    workflow_dispatch)
+        BRANCH=${WORKFLOW_BRANCH}
+        if [ -f .build_os ]; then
+            OS_VERSION=`cat .build_os`
+        else
+            OS_VERSION=${PAYLOAD_OS}
+        fi;;
     push)
         if [ -f .build_os ]; then
             OS_VERSION=`cat .build_os`