disable the PHP binding build by default

As per messages on the list, the build for the messenger PHP binding is failing in CI. If someone isn't able to fix this then it should to be disabled by default to avoid build issues for the core and any other bindings.

There only appear to have been ~5 commits in the last couple of years for it, with 3 of those being deletions relating to proton-c changes, so another discussion on these bits might be appropriate, however the immediate situation can simply be served by disabling it by default.