Since the error handling in the setup-envtest executable main path is
built on panics, recovers and os.Exit, it isn't ideal for re-using in a
version that downloads binaries within the test run.

However, much of the heavy lifting is performed by the remote, store and
versions packages, which _do_ properly return errors, and can readily be
reused in a new implementation.

This commit just duplicates the code from under tools/setup-envtest to
pkg/envtest, where we can build a new entry point that is easier to use
from within tests.

Some options might be missing...

This also removes the last references to the old env package!

Also ensure binDir option is used in all workflows