Mix.Tasks.DepsGitTest: Write permissions for object database for Windows to clean

In `Mix.Tasks.DepsGitTest`, the "updates the lock when the repo updates" test would fail because:

1. The object database for Git repositories is read-only
2. Removing read-only files using `:file`'s functions returns `{:error, :eacces}`
3. `Mix.Tasks.Deps.Clean.run` would throw `File.Error` for these read-only files claiming "permission denied".

Issuing a `chmod` command on WIndows fixes this, but I don't know:

- why this only occurs on this one test,
- if there's a better method than issuing the `chmod` command, or
- whether it would be better to (a) modify `Mix.Tasks.Deps.Clean.run` to do this universally or (b) issue some git command that cleans its object database or make it writeable.

Hm... at least there's only one Mix test failure now! ("archive" in `Mix.Tasks.LocalTest`)