Registry keys are created in per-user HKCU instead of system-wide HKLM
This bug is related to #1515 (closed). The installer shouldn't create any keys under HKCU, and should use system-wide HKLM instead.
Now, the registry key is accessible only to the user who installed the GHC. Other users don't have the installation path in the registry, and this breaks all tools which rely on getting the installation path from the registry.