Use a deterministic mirror URL instead of CDN for aportgen. Otherwise we
may generate a pmaport that wraps an apk from Alpine (e.g. musl-armv7)
locally with one up-to-date mirror given by the CDN. But then the build
will fail if CDN picks an outdated mirror for CI or BPO.
While at it, also remove unnecessary "#!/usr/bin/env python3" in files
that only get imported, and adjust other empty/comment lines in the
beginnings of the files for consistency.
This makes files easier to read, and makes the pmbootstrap codebase more
consistent with the build.postmarketos.org codebase.