oeqa/lib/utils/postactions: fix host disk usage stats retrieval

The recently introduced postactions module  can raise, on failing ptests,
the following warning:

WARNING: core-image-ptest-glib-2.0-1.0-r0 do_testimage: Can not get host
disk usage: [Errno 2] No such file or directory: '/usr/bin/df'

The issue is likely not happening because of df absence (to be confirmed
amongst the variety of workers) but because of the wrong path. Fix it by
letting subprocess search for df, passing only the binary name. To make it
work, we also have to reset the environment, otherwise the environment
configured before running bitbake will be used, and search will fail.

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