When the `paths` argument is too big `symlinkJoin` will fail with:
```
while setting up the build environment: executing '/nix/store/rm1hz1lybxangc8sdl7xvzs5dcvigvf7-bash-4.4-p23/bin/bash': Argument list too long
```
This is fixed by passing `paths` as a file instead of as an
environment variable.
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