Offline Builds

esy supports workflow where builds should happen on a machine which is completely offline (doesn't have network access).

To do that you need to use --cache-tarballs-path option when running esy install command:

On a machine which has network access execute:
```
% esy install --cache-tarballs-path=./_esyinstall
```
this will create _esyinstall directory with all downloaded dependencies' sources.
Tranfer an entire project directory along with _esyinstall to a machine which doesn't have access to an external network.
Execute the same installation command
```
% esy install --cache-tarballs-path=./_esyinstall
```
which will unpack all source tarballs into cache.
Run
```
% esy build
```
and other esy commands.