nixos/doc: add instructions for installation behind a proxy (#45854)

The instructions to install nixos behind a proxy were not clear. While
one could guess that setting http_proxy variables can get the install
rolling, one could end up with an installed system where the proxy
settings for the nix-daemon are not configured.

This commit updates the documentation with

1. steps to install behind a proxy

2. configure the global proxy settings so that nix-daemon can access
internet.

3. Pointers to use nesting.clone in case one has to use different proxy
settings on different networks.

1 files changed, 7 insertions, 0 deletions

diff --git a/nixos/modules/system/activation/top-level.nix b/nixos/modules/system/activation/top-level.nix
index 9d410a8b9ca4..fff88e2c2bf3 100644
--- a/nixos/modules/system/activation/top-level.nix
+++ b/nixos/modules/system/activation/top-level.nix
@@ -162,6 +162,13 @@ in
       description = ''
         Additional configurations to build based on the current
         configuration which then has a lower priority.
+
+        To switch to a cloned configuration (e.g. <literal>child-1</literal>)
+        at runtime, run
+
+        <programlisting>
+        # sudo /run/current-system/fine-tune/child-1/bin/switch-to-configuration test
+        </programlisting>
       '';
     };