nix-shell --pure: let variables for Wayland through

We let DISPLAY (X11) through, so we should let the Wayland equivalents
through as well.  Similarly, we let HOME through, so it should be okay
to allow XDG_RUNTIME_DIR (which is needed for connecting to Wayland
with WAYLAND_DISPLAY) through as well.  Otherwise graphical
applications will either fall back to X11 (if they support it), or
just not work (if they don't).

src/nix-build/nix-build.cc

@@ -105,7 +105,8 @@ static void main_nix_build(int argc, char * * argv)
 
     // List of environment variables kept for --pure
     std::set<string> keepVars{
-        "HOME", "USER", "LOGNAME", "DISPLAY", "PATH", "TERM", "IN_NIX_SHELL",
+        "HOME", "XDG_RUNTIME_DIR", "USER", "LOGNAME", "DISPLAY",
+        "WAYLAND_DISPLAY", "WAYLAND_SOCKET", "PATH", "TERM", "IN_NIX_SHELL",
         "NIX_SHELL_PRESERVE_PROMPT", "TZ", "PAGER", "NIX_BUILD_SHELL", "SHLVL",
         "http_proxy", "https_proxy", "ftp_proxy", "all_proxy", "no_proxy"
     };