msmtp: add sendmail symlink to $out/bin

This is useful for applications expecting a 'sendmail' binary in $PATH,
similar how nullmailer and ssmtp do it, too.

pkgs/applications/networking/msmtp/default.nix

@@ -40,6 +40,8 @@ in stdenv.mkDerivation rec {
     substitute scripts/msmtpq/msmtp-queue $out/bin/msmtp-queue \
       --replace @msmtpq@ $out/bin/msmtpq
 
+    ln -s msmtp $out/bin/sendmail
+
     chmod +x $out/bin/*
   '';