ktest: Fix post install command

The command to run post install (for those that want initrds) was
broken. Instead of doing a substitution for the $KERNEL_VERSION
variable. It was replacing the entire command with nothing.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

tools/testing/ktest/ktest.pl

@@ -907,7 +907,7 @@ sub install {
     return if (!defined($post_install));
 
     my $cp_post_install = $post_install;
-    $cp_post_install = s/\$KERNEL_VERSION/$version/g;
+    $cp_post_install =~ s/\$KERNEL_VERSION/$version/g;
     run_command "$cp_post_install" or
 	dodie "Failed to run post install";
 }