Support workspaces in release workflow

[benfrankel]

Provide an env variable (off by default) to specify a workspace sub-directory so that the release workflow can use that and Just Work.

[benfrankel]

Currently this should Just Work as long as you:

1. Set assets_path to path/to/assets if needed.
2. Keep Trunk.toml in the root directory, with build.target pointing to path/to/web/index.html if needed.
3. Make sure cargo build with no args will build the "main app" that you want to release. This can be done by setting the main app as the root package or one of the default-members, and setting cargo_build_binary_name if needed.

This should be explained in the workflows.md documentation.

We could still add an optional cargo_build_package env variable to be passed to cargo via --package, in case the user doesn't want to set the main app as the root package or build all of the default-members during release. However, trunk does not support --package currently (see trunk-rs/trunk#719).