mandoc: set MANPATH_BASE additionally to MANPATH_DEFAULT

MANPATH_BASE is used by mandoc -T lint to check if man pages that are
referenced exist on the system. While this feature is a bit problematic
for us (e. g. it will never work correctly in the nix sandbox), it
hasn't even worked in an impure context before since MANPATH_BASE
defaults to /usr/share/man:/usr/X11R6/man.

pkgs/tools/misc/mandoc/default.nix

@@ -32,6 +32,7 @@ stdenv.mkDerivation rec {
 
   configureLocal = ''
     MANPATH_DEFAULT="/run/current-system/sw/share/man"
+    MANPATH_BASE="$MANPATH_DEFAULT"
     OSNAME="NixOS"
     PREFIX="$out"
     LD_OHASH="-lutil"