Add missing space in cherry_pick.sh (pantsbuild#19026)

> GraphQL: Could not resolve to a User with the login of
'thejcannonchrisjrn'. (u000)

Another 🤦

build-support/bin/cherry_pick.sh

@@ -62,7 +62,7 @@ if [[ -z $CATEGORY_LABEL ]]; then
   read -r CATEGORY_LABEL
 fi
 REVIEWERS=$(gh pr view "$PR_NUM" --json author --jq '.author.login')
-REVIEWERS+=$(gh pr view "$PR_NUM" --json reviews --jq '.reviews.[].author.login' | sort | uniq)
+REVIEWERS+=" $(gh pr view "$PR_NUM" --json reviews --jq '.reviews.[].author.login' | sort | uniq)"
 
 BODY_FILE=$(mktemp "/tmp/github.cherrypick.$PR_NUM.XXXXXX")
 gh pr view "$PR_NUM" --json body --jq '.body' > "$BODY_FILE"