oeqa/utils/commands.py: Make a copy of variables in get_bb_vars

oeqa/utils/commands.py: Make a copy of variables in get_bb_vars

The function get_bb_vars will remove items for the list passed
as the function argument, this will leave the caller with an
empty list and the function never says it will consume the items.

This hasn't been found before because only get_bb_var uses this
function.

(From OE-Core rev: 22b7fa24fefcc3974806d1b282c93b8c5880f6a4)

Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>