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

This is the semantics as understood by `xdg-open`. Using these semantics
on a non-colon-separated variable works because it acts as if it was a
one element long list.

This fixes an issue where it would try to exec
`google-chrome-beta:google-chrome:chromium:firefox` on a system
configured with these semantics in mind.

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