Support --manifest-path for specifying a project dir

[ryankurte]

when building components of a larger project it is often useful to specify the manifest directory directly, particularly because we can't really do multi-target workspaces rust-lang/cargo#7004.

relates to actions-rs/cargo#86 which could mitigate this problem for github-actions.

(also seem to be seeing a similar issue to rust-lang/cargo#4866 when trying to build log for the wrong target when using a workspace but, not sure why that is)

[PhilippMolitor]

currently experiencing the same issue. should be as easy as adding the arg to src/lib.rs and then passing it correctly to cargo in the cargo_build_args function.

[PhilippMolitor]

can be closed