openbsd.make-rules: Fix hardcoded /bin/pwd

OpenBSD loves hardcoding paths to programs.
The only example of this in `bsd.obj.mk` is `/bin/pwd`,
so `substituteInPlace` is fine.

Co-Authored-By: Audrey Dutcher <audrey@rhelmot.io>

pkgs/os-specific/bsd/openbsd/pkgs/make-rules/package.nix

@@ -34,6 +34,8 @@ mkDerivation {
     sed -i -E \
       -e 's|/usr/lib|\$\{LIBDIR\}|' \
       share/mk/bsd.prog.mk
+
+    substituteInPlace share/mk/bsd.obj.mk --replace-fail /bin/pwd pwd
   '';
 
   installPhase = ''