Set ManifestDPIAware to true, so that the installers don't look blurry on Windows machines with high-DPI screens and scaling turned on.

Summary: Without ManifestDPIAware set to true, the NSIS installers created will be scaled by Windows in a very hamfisted fashion that leaves them looking like one needs to put on glasses. Contemporary versions of NSIS can draw just fine on high-DPI screens when given the opportunity, and there aren't any graphical changes from the stock NSIS installer theme to complicate this, so might as well enable it! See https://nsis.sourceforge.io/Reference/ManifestDPIAware for (slightly) more information.

Reviewers: Craft, vonreth

Reviewed By: Craft, vonreth

Subscribers: vonreth

Differential Revision: https://phabricator.kde.org/D17979