standardize on amd64 for x86_64 artifact names

[shmick]

The 64bit linux artifact has the suffix x86_64 while the rest of the artifacts use amd64

dnscrypt-proxy-linux_x86_64-2.1.1.tar.gz
dnscrypt-proxy-netbsd_amd64-2.1.1.tar.gz

When using utils like docker buildx, amd64 is the architecture type for x86_64 and amd64, and exposes that as the TARGETARCH environment variable.

If we try to download the build artifact using something like:

URL="https://github.com/jedisct1/dnscrypt-proxy/releases/download/${VERSION}/dnscrypt-proxy-linux_${TARGETARCH}-${VERSION}.tar.gz"

This will fail since TARGETARCH = amd64 and dnscrypt-proxy-linux_amd64-2.1.1.tar.gz does not exist.

[jedisct1]

This is unfortunate that the architecture is sometimes called x86_64 and sometimes called amd64.

Linux calls it x86_64. But on BSD, the architecture is called amd64:

$ arch
OpenBSD.amd64

The package name matches what people are used to for their operating system.