Build Images jobs on CI broken due to unexpected Github change

[Ash Berlin-Taylor]

A bit of a PSA: GitHub have made "a change" and broken our CI jobs (and 
lots of other people)

For more info and cathartic complaining see these two
<https://github.com/actions/runner/issues/1532>

<https://github.community/t/breaking-change-to-concurrency-group-syntax/215604/9>

-ash