check_php_build.php

Block running under php8 to reduce user reported issues.

phpIPAM is broken by backward incompatible changes introduced in php8.0.
Unable to investigate further as the VSCode php language server also constantly
crashes under php 8 and xdebug doesn't work reliably either.

php8 support won't be ready until at-least v1.6 so block for now.
This change be reverted in `master` after the 1.5 release when php 8 fixes are ready.

Closes #3232
Closes #3325
Closes #3326