`up`/`custom`: add support to add binaries to the environment in custom operations

[xaf]

We have binaries saved as part of the environment for most operations, which allows to make sure we load them for the current repository. At the moment, doing this for a custom operation requires making the binary available in a directory in the repository, and then adding this path to the PATH using the env: parameter so it can be used.

It would be nice to:

• Provide an "install" directory
• Copy the contents of that install directory in the stable path for the repository, which is going to persist those binaries, and make them available through the shims too