CI: prevent Infection plugins during build time, as we do not use it

[keradus]

to prevent this error on CD:

  infection/extension-installer contains a Composer plugin which is blocked by your allow-plugins config. You may add it to the list if you consider it safe.                           
  You can run "composer config --no-plugins allow-plugins.infection/extension-installer [true|false]" to enable it (true) or disable it explicitly and suppress this exception (false)  
  See https://getcomposer.org/allow-plugins  

[coveralls]

coverage: 94.521%. remained the same
when pulling 647d346 on cd
into b7b29b8 on master.