CI testing and release build for TeX Live

The binaries for TeX Live are built using Github actions, using the action defined in this repository.

Currently, the following architectures are built

• x86_64-linux: built using CentOS 7 (Glibc 2.17) with Gcc 9 via centos-release-scl and devtoolset-9
• i386-linux: built using Ubuntu Xenial (Glibc 2.23) since the above method is not available for i386 and the standard Gcc compiler in CentOS 7 (gcc4) cannot compile harfbuzz
• x86_64-linuxmusl: built using Alpine 3.5 (3.2 was used before, but we need newer compiler for harfbuzz again)