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>
diff --git a/tools/testing/ktest/ktest.pl b/tools/testing/ktest/ktest.pl
index 8ce792e..7301d03 100755
--- a/tools/testing/ktest/ktest.pl
+++ b/tools/testing/ktest/ktest.pl
@@ -907,7 +907,7 @@
     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";
 }