init: Add environment variable for git-lfs

Convenient way to always enable or disable git-lfs without having to
remember to put on the command line.

Useful if you want to ALWAYS have git-lfs enabled on your system when
you 'init' a new project.

Also useful if you are using the Jenkins repo plugin as it doesn't
provide an option for enabling git-lfs in its UI.

Change-Id: Ieb1bbe83de9c21523ab69b30fc5047c257d02731
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/437661
Commit-Queue: Scott Lee <ddoman@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Fatahillah Wk <fatahillahwkwk@gmail.com>
Reviewed-by: Scott Lee <ddoman@google.com>
Tested-by: Matt Moeller <moeller.matt@gmail.com>
Reviewed-by: Yingchun Li <sword.l.dragon@gmail.com>

subcmds/init.py

@@ -127,6 +127,7 @@ to update the working directory files.
         return {
             "REPO_MANIFEST_URL": "manifest_url",
             "REPO_MIRROR_LOCATION": "reference",
+            "REPO_GIT_LFS": "git_lfs",
         }
 
     def _SyncManifest(self, opt):