Use `gitea/test_env` image instead of `golang` (#23455)
The `safe.directory` setting was not executed for pull requests, which
made subsequent `deps-backend` target fail at `go mod download`. To fix
it, split thep and perform the git config unconditionally.
Example: https://drone.gitea.io/go-gitea/gitea/69477/4/3
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
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