Circle CI: Add Linux AArch64 job

[kinke]

No description provided.

[kinke]

[Working as intended, i.e., the failures are legit without hackily disabling these remaining ones, as we do for the Cirrus aarch64 job.]

[kinke]

Side note: the problem with CircleCI is that they impose a monthly limit (30k credits) for the free open-source plan, so it's not suited for 'deployment' jobs (uploading artifacts to GitHub), incl. superseding the Cirrus AArch64 job, which has been problematic in the last few weeks (hopefully just a bug: cirruslabs/cirrus-ci-docs#1196 (comment)).

macOS jobs are about 4x as expensive as Linux ones (75 credits per minute vs. 20, for the resource class with 4 CPU cores: https://circleci.com/product/features/resource-classes/), so we might want to get rid of at least one of the 2 macOS jobs.

[kinke]

Erm nope, the limit is fortunately much higher, I missed the open-source specific part:

CircleCI offers free access to compute time and resources for organizations on our Free plan. Open source projects can access up to 400,000 credits per month (equivalent to 80,000 build minutes) to use on Linux, Arm, and Docker.

Open source Windows and macOS builds or private repositories can use the 30,000 credits offered in our free plan.