Make sure index public key file name ends with .rsa.pub

[bgermann]

When melange build --signing-key key.rsa is called, it should make sure that the index's public key name ends with .rsa.pub.
With the current implementation, if the file argument does not end with .rsa, it will still successfully generate an APKINDEX.tar.gz that does not have the correct name. Also, melange keygen should probably make sure that the key file names end with .rsa and .rsa.pub so that you do not run into issues further on.

[bgermann]

When I tried building a package with the Adélie Linux repository, this problem also hits because the pubkey in their APKINDEX.tar.gz does not end on .rsa.pub but on .org.pub

[tuananh]

someone already submitted a PR to fix this: chainguard-dev/go-apk#226