Cirrus: Use per-project caches

[JeremyRand]

I'm seeing this in the plain-binaries task:

[13:17:09.603] SHA for cache folders (/tmp/cirrus-ci-build/out) is '1d984823efc7aa958bbde08b075d67ee82d9a3a723cce6bf92081d81893bd862'
[13:17:09.603] Cache out_release_linux_x86_64 has changed!
[13:17:09.603] List of changes for cache folders (/tmp/cirrus-ci-build/out):
[13:17:09.603] created: plain-binaries/ncdns-c947efb679dd-linux-x86_64-2241e6.tar.xz
[13:17:09.603] created: q/q-0.0.4-linux-x86_64-126875.tar.gz
[13:17:14.556] out_release_linux_x86_64 cache size is 975Mb.
[13:17:14.556] Uploading cache out_release_linux_x86_64...

This cache upload took 27 seconds. Seems like it'd be faster if we only had to upload the output folders for the projects that actually got a change.

We should be able to do this by making the YML generator iterate over each of the subfolders of projects. The YML will need to be regenerated when new projects are added, but now that we verify determinism of the YML, Cirrus will remind us to do so.