Linux: do not mount feedstock in CI to avoid cache copies, only locally

[jaimergp]

Comment:

Mounting the feedstock means that the existing cached files in the Docker image cannot be hardlinked, only copied, because it's a different filesystem. We could choose to not mount but copy while on CI to avoid that overhead (we would still do it locally). That said, that might mean issues with space available, and having to reconfigure the space available for the Docker storage. Maybe there's a flag somewhere to use a different filesystem or something?