GitHub: Don't test x32 ABI The ubuntu-20.04 hosted runner image is going away on April 1, and newer versions of Ubuntu can build but not run x32 binaries. The x32 ABI seems to be mostly dead, but we can still regression test x32 support using a local Ubuntu 20.04 VM if necessary. (That shouldn't be necessary unless the x86-64 SIMD extensions change at some point in the future.)