[infra] gitignore gclient files

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: #60848
Change-Id: I41f26b74838977d7f9fe655def8bd0c85bef7ee2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/432840
Auto-Submit: Daco Harkes <[email protected]>
Commit-Queue: Daco Harkes <[email protected]>
Reviewed-by: Hossein Yousefi <[email protected]>

Author: dcharkes

.gitignore

@@ -118,3 +118,8 @@ logs/results.json
 .dart_tool/bisect_dart/
 doc/api/
 runtime/tools/heapsnapshot/.dart_tool
+
+# gclient specific files
+.cipd
+.gclient
+.gclient_*