Cache BOUT++ build on CI

[ZedThree]

We should build BOUT++ separately and cache the built artefacts. This would reduce the build time on CI significantly.

[dschwoerer]

Or we might use a BOUT++ container - that should also make it easy to reproduce issues locally.
And we do not even have to build it the first time round, as it is already build + tested by BOUT++.

Currently that is however blocked by boutproject/BOUT-dev#3121