rc man: Make the use of $MANPATH portable

rc/tools/man.kak

@@ -55,7 +55,7 @@ define-command -hidden -params 2..3 man-impl %{ evaluate-commands %sh{
 
 define-command -params ..1 \
     -shell-script-candidates %{
-        find /usr/share/man/ $(echo $MANPATH | sed 's/:/ /') -name '*.[1-8]*' | sed 's,^.*/\(.*\)\.\([1-8][a-zA-Z]*\).*$,\1(\2),'
+        find /usr/share/man/ $(printf %s "${MANPATH}" | sed 's/:/ /') -name '*.[1-8]*' | sed 's,^.*/\(.*\)\.\([1-8][a-zA-Z]*\).*$,\1(\2),'
     } \
     -docstring %{
         man [<page>]: manpage viewer wrapper