[R] Build linux binaries on older image (like manylinux2014)

ARROW-16752 observed that even with newer compilers installed on centos 7, you can't use binaries built on ubuntu 18.04 because ubuntu 18.04 has glibc 2.27 but centos 7 only has 2.17. But if we built the binaries on centos 7 with devtoolset-7 or 8 or something, all features could compile and we'd work with older glibc.

Things built against older glibc are guaranteed to work with newer versions, and you can't just upgrade glibc because it would break the system. So for maximum compatibility, build with the oldest glibc. This strategy is like how they python manylinux standards work (IIUC).