ci: Fetch python3 from Alpine v3.14 instead of edge
Created by: jhchabran
Edge recently updated python3, which introduces a conflict on which libffi to use.
#7 3.164 ERROR: unable to select packages:
#7 3.186 libffi-3.3-r2:
#7 3.186 conflicts: libffi-3.4.2-r0
#7 3.186 satisfies: python2-2.7.18-r0[so:libffi.so.7]
#7 3.186 libffi-3.4.2-r0:
#7 3.186 conflicts: libffi-3.3-r2
#7 3.186 satisfies: python3-3.9.7-r3[so:libffi.so.8]
See https://pkgs.alpinelinux.org/package/edge/main/aarch64/python3