Merge pull request #45911 from samueldr/fix/nixos-help-browser

nixos/manual: nixos-help knows about colon-separated BROWSER

nixos/modules/services/misc/nixos-manual.nix

···
   helpScript = pkgs.writeScriptBin "nixos-help"
     ''
       #! ${pkgs.runtimeShell} -e
-      browser="$BROWSER"
+      # Finds first executable browser in a colon-separated list.
+      # (see how xdg-open defines BROWSER)
+      browser="$(
+        IFS=: ; for b in $BROWSER; do
+          [ -n "$(type -P "$b" || true)" ] && echo "$b" && break
+        done
+      )"
       if [ -z "$browser" ]; then
         browser="$(type -P xdg-open || true)"
         if [ -z "$browser" ]; then