Find a way to use a GitHub Actions cache to avoid downloading the frame files

[titodalcanton]

We download a lot of GW data from GWOSC as part of the CI tests. This fails more often than I would like - it just did. Ideally we could use a GitHub Actions cache to keep the GW data, as we now do with the ROM files for LALSimulation. One difficulty is that the scripts that download the frame files (and as a result, the frame files themselves) are scattered in a few different places, and they would need to be told not to download a frame file that already exists.