fix(build_sys): install the intl PHP extension

[JohnVillalovos]

Upstream made the intl PHP extension a requirement[1]. Update the build script to install it.

1. In commit ccd90236dc64645f3889e4efb684493dde68c949 in LibreBooking: LibreBooking/librebooking@ccd9023

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[colisee]

Thank you @JohnVillalovos
LGTM

[colisee]

@JohnVillalovos , I am afraid I should have checked the logs of the last develop image build before validating the change. It looks like composer install fails:

Root composer.json requires PHP extension ext-intl * but it is missing from your system. Install or enable PHP's intl extension.

I need to understand what's wrong here. An alternative (too be considered last) is to use composer install --ignore-platform-req=ext-intl

[colisee]

@JohnVillalovos , forget my stupid previous comment!
The explanation for the failure was that this morning scheduled action ran against the new upstream (with the new intl composer requirement) while the script build_sys.sh was still pending my approval!