Disable CGO for improved compatibility across distros

[gpassini]

This PR disables CGO so we don't have any dependencies on C libraries expected to be found in the environment, in order to achieve a better compatibility across distros.

Tested by running make install before and after the changes and inspecting the produced binary from a Linux machine (Debian in this case).
Before

ldd $(which grpcui)
	linux-vdso.so.1 (0x00007ffe2cb2e000)
	libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007fcc13946000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fcc13924000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fcc13750000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fcc1396a000)

After

ldd $(which grpcui)
	not a dynamic executable

Follow-up of the same changes in grpcurl: fullstorydev/grpcurl#420