chore: update to use public env var for preview deploy

Author: wackerow

.env.example

@@ -31,7 +31,7 @@ NEXT_PUBLIC_MATOMO_URL=
 NEXT_PUBLIC_MATOMO_SITE_ID=
 
 # Used to avoid loading Matomo in our preview deploys
-IS_PREVIEW_DEPLOY=false
+NEXT_PUBLIC_IS_PREVIEW_DEPLOY=false
 
 # Build pages only for the specified langs. Leave it empty to build all the langs
 # e.g. `en,fr` will only build English and French pages

docs/ab-testing.md

@@ -165,7 +165,7 @@ NEXT_PUBLIC_MATOMO_SITE_ID=4
 MATOMO_API_TOKEN=your_api_token_here
 
 # Preview mode flag
-IS_PREVIEW_DEPLOY=false
+NEXT_PUBLIC_IS_PREVIEW_DEPLOY=false
 ```
 
 ## Best Practices