Add an X86_64 build for linux

[adam-devel]

While writing a PKGBUILD file to install this program in my arch linux system, which uses glibc (a commonly used standard library), runs on the x86_64 CPU architecture (also very commonly used), I took a glance at the releases page and it seems the only available builds for my architecture (x86_64) are built for the darwin kernel or the linux kernel with the musl standard library.

i will compile the program now, but it would be nice to have a prebuilt binary version for the x86_64 architecture running the linux kernel with the gnu c standard library.

[adam-devel]

mcfly is actually available as a prebuilt binary package in the official repos... didn't notice, checking the aur and writing a PKGBUILD has become almost second nature.

[cantino]

Happy to accept additions to the build actions.