Set env-2 for Content Publisher Tag Manager preview
Set env-2 for Content Publisher Tag Manager preview

My previous commit in 6839a9bb did not fix the tag manager 404 problem as it also needed the gtm_preview value that I didn't notice.

Before:

```
$ curl -sI https://www.googletagmanager.com/gtm.js\?id\=GTM-NQXC4TG\&gtm_cookies_win\=x\&gtm_auth\=sxvBI4QvwgTRX5e76vdIHA\&gtm_preview\=env-7 | grep HTTP
HTTP/2 404
```

After:

```
$ curl -sI https://www.googletagmanager.com/gtm.js\?id\=GTM-NQXC4TG\&gtm_cookies_win\=x\&gtm_auth\=sxvBI4QvwgTRX5e76vdIHA\&gtm_preview\=env-2 | grep HTTP
HTTP/2 200
```
alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet