oe-selftest: Improve BUILDDIR environment handling

oe-selftest: Improve BUILDDIR environment handling

Its possible something (like bitbake/tinfoil2) may mess around with the
environment and using the enviroment as a global variable store isn't
particularly nice anyway.

This patch changes the BUILDDIR usages so that the environment isn't used
as a global store and a global variable is used instead. Whilst that
is still not perfect, it does avoid the current double and triple backtraces
we're seeing where tinfoil2/bitbake has trampled the enviroment leading
to failures of failures making debugging even harder.

(From OE-Core rev: 689b676bbf2f1a5fadb04aeb41d5e68e35356545)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>