git: respect programs.msmtp.package

Since the user may override `programs.msmtp.package` using
`pkgs.msmtp` as the sendmail binary might result in the wrong package
being used.

(cherry picked from commit 258db1d39b)

modules/programs/git.nix

@@ -379,7 +379,7 @@ in
               lib.nameValuePair "sendemail.${name}" (
                 if account.msmtp.enable then
                   {
-                    sendmailCmd = "${pkgs.msmtp}/bin/msmtp";
+                    sendmailCmd = lib.getExe config.programs.msmtp.package;
                     envelopeSender = "auto";
                     from = "${realName} <${address}>";
                   }