Upgrade pip when doing CI
Try to fix the flaky tests by upgrading pip before installing dependencies.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6234?src=pr&el=h1) Report
> Merging [#6234](https://codecov.io/gh/huggingface/transformers/pull/6234?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/57eb1cb68d1c567b25ac256444e5c1a77b8817a7&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6234/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6234?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #6234   +/-   ##
=======================================
  Coverage   79.29%   79.29%           
=======================================
  Files         146      146           
  Lines       26597    26597           
=======================================
  Hits        21089    21089           
  Misses       5508     5508           
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6234?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6234?src=pr&el=footer). Last update [57eb1cb...243c98f](https://codecov.io/gh/huggingface/transformers/pull/6234?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
