qt-build-utils can invoke qmake when it is a script

[SanderVocke]

Ensure qmake is called either through cmd (Windows) or sh (Linux/MacOS).

The main change is this invocation. The rest of the code change is:

• There were two places qmake were previously invoked and they didn't share code. To prevent duplicating this shell wrapper, refactored a bit to ensure they both call the same method to query qmake.
• Due to said refactoring, also had to change the error handling a bit because the error generation moved. Added error types.

[SanderVocke]

I only tested this so far on the "happy case" on Windows (that is to say: when using a qmake .bat wrapper). Will test on other platforms too.

[SanderVocke]

I marked as ready for review.
Tested all combinations of QMAKE=qmake6 and QMAKE=(a qmake wrapper).[sh|bat] on Windows, Ubuntu and MacOS GitHub runners and cxx-qt builds fine with these combinations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (cb64b22) to head (7db4d04).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1284   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           75        75           
  Lines        12772     12772           
=========================================
  Hits         12772     12772           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[SanderVocke]

In the refactored qt-build-utils, the change was a lot more straightforward.

[SanderVocke]

@ahayzen-kdab is this still on your radar?