Why GAMMARAY_PROBE_ONLY_BUILD implies skipping install?

[Kentzo]

What is the principal reason for skipping installation of headers, pri and cmake files in probe-only builds? In my fork I allowed this installation and nothing bad happened, generated files seem appropriate.

Take a look at #929, I wonder if this extra variable is necessary at all.

[Kentzo]

My particular problem is that header for the static probe will never get installed.

[Kentzo]

It's feasible to resolve this installation issue + provide better linking experience for static plugins that avoids manual [Q_IMPORT_PLUGIN](https://doc.qt.io/qt-6/qtplugin.html#Q_IMPORT_PLUGIN) (at least for Qt6 users).

I will make a PR once all preliminary changes get merged into the project.