Conditionally run Julia in CI workflow

[epatters]

Testing Julia code in CI is always slow, as can be seen since #1292. We should cache the results of Julia tests, like we do for other build actions, so that we can avoid running them when possible.

[quffaro]

After a discussion with @epatters, we've decided to just run the Julia CI whenever files in the packages/algjulia-interop subdir have changed.