symlinkJoin: fix "bash: Argument list too long" error

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.