Build release artifacts in GitHub Actions

[Travis Ralston]

building them on the current release hardware appears to cause problems for oldstable Debian:

root@debian-s-1vcpu-512mb-10gb-sfo3-01:~# ./media_repo-linux-x64 -version
./media_repo-linux-x64: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by ./media_repo-linux-x64)
./media_repo-linux-x64: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by ./media_repo-linux-x64)