Replace dl-cdn mirror with dl-2 temporarily to avoid "BAD signature"
errors.
This is related to the rebuild of Alpine edge x86, armhf, armv7 packages
against musl-1.2, that is currently going on. The packages are rebuilt
without a version change and therefore have the same resulting file
name, but a different checksum. Due to caching of the CDN and due to the
same file name, users may get an old package from before the rebuild.
The APKINDEX has a checksum of the new package, and so the download will
fail with a "BAD signature" error.
Alpine developers clear the cache every now and then, but this issue
will happen over and over again, until all packages are rebuilt (some
packages are still not rebuilt and have been disabled).
Let's switch back in two months or so.
78f43d254e