oeqa/utils/postactions: isolate directory creation in dedicated action

oeqa/utils/postactions: isolate directory creation in dedicated action

In order to be able to create actions that could store new files during
failed test post actions, we need to split artifacts directory creation
from artifacts retrieval.

Create a new dedicated action to create artifacts main directory so we can
add actions creating files in this new directory, without worrying about
actions order if at least this action is set first.

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>