ci: fix staging version string (#2582)

* ci: fix STAGING version competition

* ci: replace slashes with dots in branch name for STAGING_VER

* Apply suggestion from @wuhuizuo

Author: wuhuizuo

.github/workflows/release-tiup.yaml

@@ -65,7 +65,7 @@ jobs:
         run: |
           BRANCH_NAME=$(git rev-parse --abbrev-ref HEAD)
           if [ "$BRANCH_NAME" != "master" ]; then
-            STAGING_VER=$(git describe --tags | sed 's/-[0-9]*-[^-]*$//')-$BRANCH_NAME
+            STAGING_VER=$(git describe --tags | sed 's/-[0-9]*-[^-]*$//')-$(echo $BRANCH_NAME | sed 's|/|.|g')
           else
             STAGING_VER=$(git describe --tags | sed 's/-[^-]*$//' | sed -r 's/(-[^-]*$)/-nightly\1/')
           fi