build-community.sh: Fixed incorrect tag detection logic in repo init.

  - Previously, the API search used substring matching, causing false positives (e.g., 'a14' incorrectly matched 'v2.8-beta.2-a14').
  - Now, we fetch all tags and use exact matching (`grep -Fx`) to determine if BRANCH_NAME is a tag.

Signed-off-by: Jackeagle <jackeagle102@gmail.com>