mpack: Invoke /run/wrappers/bin/sendmail via execvp

Calling /run/current-sw/bin/sendmail fails under postfix because
setgid bits are not set. Switching the hardcoded path to an invocation
via execvp should cover both cases, when the sendmail binary is
setgid-wrapped and when it is not.

pkgs/tools/networking/mpack/default.nix

@@ -8,18 +8,13 @@ stdenv.mkDerivation rec {
     sha256 = "0k590z96509k96zxmhv72gkwhrlf55jkmyqlzi72m61r7axhhh97";
   };
 
-  patches = [ ./build-fix.patch ];
+  patches = [ ./build-fix.patch ./sendmail-via-execvp.diff ];
 
   postPatch = ''
     for f in *.{c,man,pl,unix} ; do
       substituteInPlace $f --replace /usr/tmp /tmp
     done
 
-    for f in unixpk.c ; do
-      substituteInPlace $f \
-        --replace /usr/sbin /run/current-system/sw/bin
-    done
-
     # this just shuts up some warnings
     for f in {decode,encode,part,unixos,unixpk,unixunpk,xmalloc}.c ; do
       sed -i 'i#include <stdlib.h>' $f