Also moved `winbuild.ps1` => `windows/manual_winbuild.ps1` to indicate the
purpose of this script better.

The Windows build script is updated to use the newly compiled Boost 1.77.0.

* A minimal path is set in the beginning to avoid any unintended conflicts with
other libraries installed on the system.
* Timers are added to measure the elapsed times for building and testing.
* Verbosity of CPack is reduced.

Qt 5.15.0 LTS is installed on the Win10 VM. A directory symlink is made to
refer to the current version folder.

A Powershell script is added that can be executed under MS Windows to perform
the build steps.

* The script can be executed in Windows CLI, independent of GitLab.
* Powershell variables are used instead of the cmd variables; see
<https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_variables?view=powershell-7.1>.
* Powershell commands are simplified.
* Diagnostics are added for better build debugging.
* ccache is disabled, as it does not perform as expected.
* MSBuild flags are set for a parallel build.
* Descriptive comments are added.

The CI/CD pipeline is tested on both Windows build-machines.