mender-workardounds.bbclass: add MENDER_MACHINE workaround

mender-setup.bbclass adds an export of MENDER_MACHINE
and adds that variable BB_HASHBASE_WHITELIST, which
alters the signature for all tasks in the build. However,
that variable is only needed for their test scripts and
in meta-mender-qemu, neither of which we're using.
So to make it possible to share sstate between Mender
and non-Mender builds, undo those settings.

Signed-off-by: Matt Madison <matt@madison.systems>