create-databases: Use scap to run php scripts
Use scap php and scap mwscript to run install.php and the
upgrade maintenance scripts. This allows these scripts to work
(modulo other issues) using containerized PHP 8 even though the
train-dev deploy host is still on PHP 7.4, similar to the state of the
production deployment server.
Depends-On: scap!859 (merged)
Depends-On: scap!871 (merged)