Key CI cache on build profile (#10333)
We are aware that "lint debug ubuntu" and "test debug ubuntu" are sharing
caches - and that it is not ideal. Likely the lint cache will be written
first, and test-debug will have to build extra. However neither are the
bottleneck now.
We proceed on the assumption that we're hitting the 5 GB cache limit on a
single PR, inducing a forever-rolling behavior.
If "test debug ubuntu" becomes the bottleneck in the future we will
revisit.
