Upgrade pip ci
The SHA failures that made the tests flaky the last few weeks seem to come from a mismatch of SHA signatures from the pip cache and downloaded package, which may be due to incomplete/failed downloads. This PR removes the cache usage, which does not slow down the test and makes them more robust.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6235?src=pr&el=h1) Report
> Merging [#6235](https://codecov.io/gh/huggingface/transformers/pull/6235?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5deed37f9f1a0f5794a2a7cd02164ff265c59524&el=desc) will **increase** coverage by `1.33%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6235/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6235?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6235      +/-   ##
==========================================
+ Coverage   78.83%   80.16%   +1.33%     
==========================================
  Files         146      146              
  Lines       26597    26597              
==========================================
+ Hits        20968    21322     +354     
+ Misses       5629     5275     -354     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6235?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6235/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `71.87% <0.00%> (-23.44%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6235/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.25% <0.00%> (+1.29%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6235/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+3.25%)` | :arrow_up: |
| [src/transformers/tokenization\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/6235/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxuZXQucHk=) | `90.09% <0.00%> (+23.42%)` | :arrow_up: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6235/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `93.80% <0.00%> (+25.66%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6235/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.78% <0.00%> (+34.38%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6235/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `95.13% <0.00%> (+74.65%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6235?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6235?src=pr&el=footer). Last update [5deed37...302e6ae](https://codecov.io/gh/huggingface/transformers/pull/6235?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>FYI, CI continues to be super-flakey, failing very often on `pip install`