nixos/nginx: use mailcap mimetypes in all cases (#83611)

In ff0148d8, nginx configuration was modified to use mime.types
from mailcap package as it is more complete. However, there are two
places where mime.types is included in configuration. When the user
was setting `cfg.httpConfig`, the mime.types from nginx was still
used. This commit fix that by moving the common snippet in a variable
of its own and ensure it is used at both places.