When using

```
solutions = [
  {
    "name": ".",
    "url": "https://dart.googlesource.com/sdk.git",
    "deps_file": "DEPS",
    "managed": False,
    "custom_deps": {},
    "custom_vars": {
      "dart_root": ".",
    },
  },
]
```

the .gclient and .cipd files end up in the root of the SDK.
So, gitignore them.

Bug: https://github.com/dart-lang/sdk/issues/60848
Change-Id: I41f26b74838977d7f9fe655def8bd0c85bef7ee2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/432840


Auto-Submit: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Hossein Yousefi <yousefi@google.com>