Keeping the CI build working

[ajlennon]

The first issue is that the CI build seems to

• no longer re-use existing artifacts so builds from scratch and takes a long time
• now seems to timeout after a number of hours

I

• updated the Yocto docker build image to use Ubuntu 20.04 instead of 18.04 to use newer git which may help with a warning I was seeing with GitHub git checkout
• am looking at whether this helps checkout action clean key to work
• am looking at adding a timeout key to ensure build runs for long enough

[ajlennon]

Have updated to 22.04 now and Kirkstone is building

[ajlennon]

CI generally seems to be a lot better now so am closing this