ci: Increase number of build slaves to 7
ci: Increase number of build slaves to 7

Since builds have drastically increased in time, we end up timing out CI
if we have a larger number of tests to build.  Try bumping up the number
of slaves to 7 and see if that helps.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

punitvara/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,kraj/zephyr,finikorg/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,explora26/zephyr,kraj/zephyr,kraj/zephyr,galak/zephyr,kraj/zephyr,punitvara/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,explora26/zephyr,galak/zephyr,galak/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr