analytics_src is hardcoded, should be an env variable

[dannymandel]

Currently, when we generate the config.js as part of the build process, the analytics pieces look like this:

\"analytics_src\": \"https://metrics.isample.xyz/js/plausible.js\",\n  \"analytics_domain\": \"$ANALYTICS_DOMAIN\"

The analytics_src should be configurable the same way that the analytics_domain is, otherwise we'll always be hardcoded to the same src file (and IIRC under the hood this determines the API endpoints).