Look for gtag env var
Look for gtag env var

- modify the GA4 code to check for the existence of a gtag environment variable alongside the existing code to check for a GTM env var
- if the gtag env var is found, this will be used in preference to GTM by the code called in the components gem from window.GOVUK.analyticsGA4.init()
- otherwise will default back to GTM for all environments, or simply do nothing if no GTM or gtag env vars are found

alphagov/static,alphagov/static,alphagov/static