CI Fixes

[Kerilk]

This fixes two Issues we've been having recently withe the CI:

• MacOS images are sometimes missing gcc-9, (seems to be transitioned out) so this checks for it and installs it if required.
• Ubuntu 18.04 is deprecated, but as we run in a container we can safely update the Ubuntu version we are using so I bumped it to 20.04

[bashbaug]

LGTM. We should probably merge this as-is and ASAP to keep CI clean.

Since I think we'll need a similar fix elsewhere though (OpenCL-CLHPP, OpenCL-ICD-Loader, probably OpenCL-SDK), at some point we should investigate whether we can move from macos-11 to macos-latest now that we're explicitly installing gcc9.

[Kerilk]

Since I think we'll need a similar fix elsewhere though (OpenCL-CLHPP, OpenCL-ICD-Loader, probably OpenCL-SDK), at some point we should investigate whether we can move from macos-11 to macos-latest now that we're explicitly installing gcc9.

Indeed. I most probably will do one for the ICD loader in the process of validating KhronosGroup/OpenCL-ICD-Loader#210