__fish_apropos: assume only /usr/bin/apropos is broken

As discovered in #12062, man-db as installed from brew is not affected
by the "read-only-filesystem" problem what makewhatis suffers from
(though users do need to run "gmandb" once).

Restrict the workaround to the path to macOS's apropos; hopefully
that's stable.

This breaks people who define an alias for apropos; I guess for those
we could check "command -v" instead, but that's weird because the thing
found by type is the actual thing we're gonna execute.  So I'd first
rather want to find out why anyone would override this obscure command.

share/functions/__fish_apropos.fish

@@ -17,6 +17,12 @@ if test (__fish_uname) = Darwin
 
     function __fish_apropos -V dir
         # macOS has a read only filesystem where the whatis database should be.
+
+        if functions -q apropos || test "$(command -v apropos)" != /usr/bin/apropos
+            __fish_without_manpager apropos "$argv"
+            return
+        end
+
         # The whatis database is non-existent, so apropos tries (and fails) to create it every time,
         # which can take seconds.
         #
@@ -32,7 +38,7 @@ if test (__fish_uname) = Darwin
             set age (path mtime -R -- $whatis)
         end
 
-        MANPATH="$dir" __fish_without_manpager apropos "$argv"
+        MANPATH="$dir" __fish_without_manpager /usr/bin/apropos "$argv"
 
         if test $age -ge $max_age
             test -d "$dir" || mkdir -m 700 -p $dir