CI: don't set BOOST_ROOT to an empty string
CI: don't set BOOST_ROOT to an empty string

This was initially added in eb76ba7031f053a3c1 to avoid picking
it up from azure/gha images.

Looks like it is no longer set for some time now though:
https://github.com/actions/runner-images/pull/540
so should no longer be needed.

The empty string makes some meson tests fail which expect a path.
A recent msys2-runtime change now lets empty env vars through to child processes
whereas before they would get filtered out, which is why this worked before.

mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson