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

[samueldr]

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.

Motivation for this change

Things done

• ✔️ Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • ✔️ NixOS
  • ⬜ macOS
  • ⬜ other Linux distributions
• ⬜ Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• ⬜ Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• ✔️ Tested execution of all binary files (usually in ./result/bin/)
• ⬜ Determined the impact on package closure size (by running nix path-info -S before and after)
• ✔️ Fits CONTRIBUTING.md.

---

[grahamc]

[grahamc@Morbo:~]$ BROWSER=foo:bar:baz:firefox:google-chromestable

[grahamc@Morbo:~]$      browser="$(
>         IFS=: ; for b in $BROWSER; do
>           [ -n "$(type -P "$b" || true)" ] && echo "$b" && break
>         done
>       )"

[grahamc@Morbo:~]$ echo $browser
firefox

[samueldr]

post-merge comment

While it seems wateful to run xdg-open if it's using the same semantics for $BROWSER, xdg-open does more than simply looking at $BROWSER.