[scripts] use better %programfiles% detection for windows

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>
[buildqtc] add --gdb-only option and refactor script

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>
[buildqtc] download and install win32 binary artifacts

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>