fix: git remote syntax

usage: git remote add [<options>] <name> <url>

    -f, --fetch           fetch the remote branches
    --tags                import all tags and associated objects when fetching
                          or do not fetch any tag at all (--no-tags)
    -t, --track <branch>  branch(es) to track
    -m, --master <branch>
                          master branch
    --mirror[=(push|fetch)]
                          set up remote as a mirror to push to or fetch from

entrypoint.sh

@@ -2,7 +2,7 @@
 
 cd "${GITHUB_WORKSPACE}"
 
-git remote add -f  "https://github.com/$1.git"
+git remote add -f upstream "https://github.com/$1.git"
 git branch ${GITHUB_REF}
 git merge --ff-only upstream/${GITHUB_REF}
 git push