Fix accidental Flake8 excludes (#55178)
Summary:
[Currently](https://github.com/pytorch/pytorch/blob/faa4da49ff6fdde8411572fe907e861e95ce2c85/.flake8#L22), our `.flake8` config file has the `exclude` pattern `scripts`. I'm guessing that this is just meant to exclude the top-level `scripts` dir from Flake8, but it also applies to the following (apparently erroneously):
- `.circleci/scripts`
- `.github/scripts`
- `test/scripts`
This PR corrects the problem by making all the `exclude` patterns (except for the wildcard `*.pyi` pattern) relative to the repository root. Also, since this PR already touches all the `exclude` lines, it also sorts them to help reduce merge conflicts when `.flake8` is edited in the future. This sorting happened to reveal that the `build` pattern was previously present twice, so now it has been deduplicated.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55178
Test Plan:
Locally:
```
flake8
```
And also [in CI](https://github.com/pytorch/pytorch/pull/55178/checks?check_run_id=2249949511).
Reviewed By: janeyx99
Differential Revision: D27520412
Pulled By: samestep
fbshipit-source-id: 359275c10ca600ee4ce7906e3a7587ffaa4ae1ed
