Copy the category label when cherry-picking (pantsbuild#16255)

This is a little bit untested, but I did test the `jq` expression on the CLI 🤞 

[ci skip-rust]
[ci skip-build-wheels]

build-support/bin/cherry_pick.sh

@@ -36,8 +36,9 @@ fi
 
 COMMIT=$(gh pr view "$PR_NUM" --json mergeCommit --jq '.mergeCommit.oid')
 TITLE=$(gh pr view "$PR_NUM" --json title --jq '.title')
+CATEGORY_LABEL=$(gh pr view "$PR_NUM" --json labels --jq '.labels.[] | select(.name|test("category:.")).name')
 BODY_FILE=$(mktemp "/tmp/github.cherrypick.$PR_NUM.$MILESTONE.XXXXXX")
-PR_CREATE_CMD=(gh pr create --base "$MILESTONE" --title "$TITLE (Cherry-pick of #$PR_NUM)" --body-file "$BODY_FILE")
+PR_CREATE_CMD=(gh pr create --base "$MILESTONE" --title "$TITLE (Cherry-pick of #$PR_NUM)" --label "$CATEGORY_LABEL" --body-file "$BODY_FILE")
 BRANCH_NAME="cherry-pick-$PR_NUM-to-$MILESTONE"
 
 if [[ -z $COMMIT ]]; then