By using ${CMAKE_COMMAND} we make sure the current cmake is used,
instead of relying that some cmake can be found in the path

I hit this issue while building Kaidan using craft,
outside of the KDE Android docker container. Hopefully this is the
correct way to fix it.

These are the variables that cmake will pass onto itself when doing a
try_compile, we need that because otherwise when trying to find iconv it
will issue a try compile, won't pass CMAKE_ANDROID_API so it will
default to 21 (instead eg the 28 we had given it) and the try_compile
will fail because android libc only has iconv after API 28

- We don't need the Threads target workaround anymore, that breaks the
build with Qt6 even.
- The Gradle wrapper shipped with Qt is no longer installed as executable,
so we need to run this in sh explicitly.
- Qt6 uses a different Android Gradle plugin version (not to be confused
with the Gradle version), which we need to make available for the
configure_file() call on the build.gradle file.

With this most Framework modules build against Qt6 here.

The relevant issue is only fixed in 23

I was seeing `error: No such remote 'origin'` in the cmake output.
This commit avoids hardcoding `origin` as the upstream URL and instead
uses the `git rev-parse @{u}` to get the configured upstream.

As a follow-up we may want to check if this should be executed by default,
but for now this fixes a warning that I'm seeing with various projects.

And the generated html looks more correct

GIT_SILENT