Fix 64-bit uninstaller (#19067)
Fixes #19029
### Summary of the issue:
The uninstaller for 64-bit builds of NVDA doesn't work.
### Description of user facing changes:
The uninstaller works again.
### Description of developer facing changes:
None.
### Description of development approach:
Tell NSIS that the NVDA install path should be retrieved from a 64-bit
view of the registry.
### Testing strategy:
Build a launcher. Install it in Windows Sandbox. Uninstall it.
### Known issues with pull request:
None.
