prefetch-yarn-deps: fix resolving of local packages using file:

Without this change local packages would get rewritten to resolve to
something that cannot be found at yarnpkg.com

Example from a package prior this change:

Excerpt from `yarn.lock`:
```
@fortawesome/fontawesome-pro@file:./pkg/fortawesome-fontawesome-pro-6.3.0.tgz
file_._pkg_fortawesome_fontawesome_pro_6.3.0.tgz
```

The `yarnConfigHook` results in:
```
> Executing yarnConfigHook
> yarn config v1.22.22
> success Set "yarn-offline-mirror" to "/nix/store/r02716lg5ric6rihpnh75lrmpdw9mwv4-offline".
> Done in 0.02s.
> yarn install v1.22.22
> [1/4] Resolving packages...
> [2/4] Fetching packages...
> error Can't make a request in offline mode ("https://registry.yarnpkg.com/file_._pkg_fortawesome_fontawesome_pro_6.3.0.tgz")
```

0 files changed, 0 insertions, 0 deletions