Platform override doesn't work

[zooba]

Tested briefly on an ARM64 machine.

py install 3.14-arm64 and py -V:3.14-arm64 both preferred the (default) x64 builds. After setting %PYTHON_MANAGER_DEFAULT_PLATFORM%=-arm64, it worked fine.

Most likely there's some code that is checking for tag+platform before tag, when it should be checking for tag and then tag+platform if nothing matched.