build: use vendored packages for circle backend tests (#19708)

scripts/circle-test-backend.sh

@@ -3,6 +3,9 @@
 # shellcheck source=./scripts/helpers/exit-if-fail.sh
 source "$(dirname "$0")/helpers/exit-if-fail.sh"
 
+# use vendor folder for packages
+export GOFLAGS=-mod=vendor
+
 echo "building backend with install to cache pkgs"
 exit_if_fail time go install ./pkg/cmd/grafana-server