Ensure Env.Host.sh always return a zero exit code

* On some NetBSD systems, the last unset command returns a
  non-zero exit code

* This made the make(1) processes terminate immediately in
  source-env-and-recurse, breaking the build.

Reported by: Thomas Klausner <wiz@NetBSD.org>

set_soenv.in

@@ -1102,6 +1102,9 @@ ToFile( "ENV_SCRIPT", "$outfile", "e" );
 #
 print OUT "export $exportvars$newline";
 print OUT "unset $unsetvars$newline";
+# unset may return a non-zero value and make the initial
+# make(1) processes terminate with an error
+print OUT "true $newline";
 
 #
 #---------------------------