ci: Add config section for allow-plugins

[Chris53897]

This will hopefully fix the ci

Error: dealerdirect/phpcodesniffer-composer-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.dealerdirect/phpcodesniffer-composer-installer [true|false]" to enable it (true) or disable it explicitly and suppress this exception (false)

[Chris53897]

I have closed this PR in favour of #296
cache@v2 does break the ci