ci(dev): add `.git/` to `.gitignore`

For some reason, some tools like [ripgrep](https://github.com/BurntSushi/ripgrep) will
search `.git` when using the `-.`/`--hidden` flag, even when not using `--no-ignore-vcs`.
This leads to several unwanted results.

.gitignore

@@ -73,3 +73,4 @@ apps/emqx_conf/etc/emqx.conf.all.rendered*
 rebar-git-cache.tar
 # build docker image locally
 .docker_image_tag
+.git/