.github/workflows/build.yml: build with GCC 10
.github/workflows/build.yml: build with GCC 10

Building with clang failed all the time due to libc++abi linking
problems (clang did not link with that library automatically), and now
I found out that GCC 10 is available in Ubuntu 21.04 (but is not the
default).

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy