CI : mikrobus greybus update & CI build fixes

buildroot issue was probably this: http://buildroot-busybox.2317881.n4.nabble.com/PATCH-1-1-pkg-cmake-mk-fix-host-ccache-support-for-CMake-3-19-td271371.html and was fixed after bumping up the version to 2020.11.1

firmware build was failing at the 'installing zephyr pre-requisites stage' due to trying to fetch a old/non existent package resulting in 404, adding a sudo apt update solves the problem.

running gbridge on qemu still fails like https://github.com/jadonk/beagleconnect/pull/37#commitcomment-43564138