[.htaccess] - Define "AuthUserFile" with an environment variable

Question

Hello everybody !

Do you know how I could do to use a variable to define the AuthUserFile in a .htaccess file ? Like :

RewriteRule ^ - [E=BASE:%{DOCUMENT_ROOT}mysite/www/]
AuthUserFile %{ENV:BASE}admin/.htpasswd

It don't work... Here is my full .htaccess file (if you want to understand why I need to do that) :

Any help would be really appreciated !