Add a `validate integrations` trigger to validate integrations without running their tests

We have the `test integrations` trigger that launches a full integrations build to ensure that a change doesn't break any of the packages whose source is there.

This integrations build is quite heavy as it runs all the system tests too, but in most cases, when making changes in the Package Spec, we are mostly interested on checking that validation doesn't break for any package.

Add a new `validate integrations` trigger that only tries to validate and build packages in the integrations repository, without running all the tests.

cc @mrodm