Existing cache for local dataset builder file updates is ignored with `ignore_verifications=True`
The cache is based on the hash of the dataset builder's code, so changing the code makes it recompute the cache.

You could still rename the cache directory of your previous computation to the new expected cache directory if you want to avoid having to recompute it and if you're sure that it would generate the exact same result.

The verifications are data integrity verifications: it checks the checksums of the downloaded files, as well as the size of the generated splits.
## Describe the bug
If i have local file defining a dataset builder class and I load it using `load_dataset` functionality, the existing cache is ignored whenever the file is update even with `ignore_verifications=True`. This slows down debugging and cache generator for very large datasets.

## Steps to reproduce the bug

- Create a local dataset builder class
- load the local builder class file using `load_dataset` and let the cache build
- update the file's content
- The cache should rebuilt.

## Expected results

With `ignore_verifications=True`, `load_dataset` should pick up existing cache.

## Actual results

Creates new cache.

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.8.0
- Platform: Linux-5.4.0-52-generic-x86_64-with-debian-bullseye-sid
- Python version: 3.7.7
- PyArrow version: 3.0.0

Existing cache for local dataset builder file updates is ignored with `ignore_verifications=True` 
 ## Describe the bug
If i have local file defining a dataset builder class and I load it using `load_dataset` functionality, the existing cache is ignored whenever the file is update even with `ignore_verifications=True`. This slows down debugging and cache generator for very large datasets.

## Steps to reproduce the bug

- Create a local dataset builder class
- load the local builder class file using `load_dataset` and let the cache build
- update the file's content
- The cache should rebuilt.

## Expected results

With `ignore_verifications=True`, `load_dataset` should pick up existing cache.

## Actual results

Creates new cache.

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.8.0
- Platform: Linux-5.4.0-52-generic-x86_64-with-debian-bullseye-sid
- Python version: 3.7.7
- PyArrow version: 3.0.0
 
 The cache is based on the hash of the dataset builder's code, so changing the code makes it recompute the cache.

You could still rename the cache directory of your previous computation to the new expected cache directory if you want to avoid having to recompute it and if you're sure that it would generate the exact same result.

The verifications are data integrity verifications: it checks the checksums of the downloaded files, as well as the size of the generated splits.