lava/submit-job.sh: skip install multidict from sources

When install lavacli, multidict libaray will be installed. In some cases, multidict is installed by compiling from sources.

It caused error like: "building 'multidict._multidict' extension multidict/_multidict.c:1:10: fatal error: Python.h: No such file or directory"

To skip compilation, adding variable MULTIDICT_NO_EXTENSIONS=1 like "MULTIDICT_NO_EXTENSIONS=1 pip3 install lavacli"

Signed-off-by: Nguyen Dat Tho tho1.nguyendat@toshiba.co.jp