docker: Fix travis script unable to find source dir

docker: Fix travis script unable to find source dir

The script generated from QEMU_SRC/.travis.yml uses BUILD_DIR and
SRC_DIR path relative to the current dir, unless these variables
are exported in environment.

Since commit 05790dafef1 BUILD_DIR is exported in the runner script,
although SRC_DIR is not, so that make docker-travis fails becase
the reference to source dir is wrong. So let's unset both BUILD_DIR
and SRC_DIR before calling the script, given it is executed from
the source dir already (as in Travis).

Signed-off-by: Wainer dos Santos Moschetta <[email protected]>
Message-Id: <20190320221207[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>