ci: fix build of arm64 docker image

[mdonadoni]

As discussed IRL, let's remove arm64 version building for now, and find a PPA to avoid building memory-huge dependencies and/or keep releasing arm64 from local laptops.

Originally posted by @tiborsimko in #66 (review)

Currently the arm64 docker image is not build automatically by the CI after a new release, as the build process fails while building the wheel of xrootd. This is probably due to exceeding the memory available on GitHub's runners.

Possible solutions:

• use a prebuilt version of xrootd from either Ubuntu's repo or a third-party PPA
• tweak the build process of xrootd to consume less memory