Instead of having a matrix of jobs, use a single job running
all the tests like the linux jobs do.
This helps with improved cache hits as most of the deps are shared
between the builds.
In gst-rs we build each crate on its own, since not all
crates share the same features and some conflict with each other.
However currently, that isn't the case in plugins-rs and instead
we can be building all the crates with the same flags and simplify
the the script.
Close#241
