Fix the Conda package build
I saw that [the Conda builds are failing since v4.12](https://github.com/huggingface/transformers/actions/workflows/release-conda.yml). The main problem is that, for some reason, [the build tries to install `setuptools` but Conda build forbids it](https://github.com/huggingface/transformers/runs/5853294684?check_suite_focus=true#step:6:2172). I found [an answer in StackOverflow](https://stackoverflow.com/a/64825075/1165181) that shows it can be fixed by adding the flags ` --single-version-externally-managed --record=record.txt` to the `python setup.py install` command in the `build.sh` file (note the `--record` flag is also needed, otherwise the command fails, stating so).

I also updated the tokenizers version specification, which seemed to have been forgotten to be updated in this file as well.

I added `conda-verify`, which `conda build` uses for some sanity checks.

Finally, I changed `conda-build` to `conda build`, which seems to be the way to use this command.

It'd be good if somebody can check this on their end, to double-check it's working fine:

```bash
conda create -n build-transformers -c huggingface python=3.8 anaconda-client conda-build conda-verify
conda activate build-transformers
TRANSFORMERS_VERSION=$(python setup.py --version) conda build .github/conda
```
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger I saw you have worked on the Conda packaging of this repo before. Can you look into it? IMHO this PR doesn't take a lot of time to review.<|||||>@LysandreJik friendly ping!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>@LysandreJik friendly ping :)<|||||>Sorry, just got to it! I managed to get the build to run correctly by just adding the `tokenizers` line :tada: Let me know if you'd like for me to send logs.

Are you sure we need the rest? I'd be happy to merge your PR with just the tokenizers changes :)<|||||>Yeah. I explained why in the PR description<|||||>Would you like me to share the logs I got locally showing only the `tokenizers` change was necessary?<|||||>> Would you like me to share the logs I got locally showing only the `tokenizers` change was necessary?

I left only the `tokenizers` change now.<|||||>The failing tests are flaky, right?