ci/lib: set TERM environment variable if not exist

GitHub Action doesn't set TERM environment variable, which is required
by "tput".

Fallback to dumb if it's not set.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/lib.sh

@@ -79,6 +79,9 @@ check_unignored_build_artifacts ()
 	}
 }
 
+# GitHub Action doesn't set TERM, which is required by tput
+export TERM=${TERM:-dumb}
+
 # Clear MAKEFLAGS that may come from the outside world.
 export MAKEFLAGS=