Refactoring of build.py and bugfixes to improve the support of Windows

[easymoney322]

I've made some changes to build.py:

• Removed consecutive slashes that would occur in some places due to path concatenation;
• Replaced cp/mv system calls on windows with shutil.copy2 and os.replace respectively. Now you can use system shells such as PowerShell or CMD to build;
• Fixed os.rename causing windows-specific error except FileExistsError if the ./rustdesk-{version}-install.exe still existed after the previous build. This also improves ACLs handling;
• Replaced system calls for python3 on Windows to use python instead, as it's the default python3 binary, and the python2 is deprecated. This should allow to build on the new systems without symlinking python3 to python;

It might be better to use some kind of logic (like getting and parsing the output of python -V and python3 -V respectively) to determine which binary to use, and then calling the binary directly, but I find it unnecessary complicated as Python 2 is deprecated anyway.

I've tested it with nightly workflow, as well as with python ./build.py --hwcodec --flutter and .\build.py --portable --hwcodec --flutter --vram --skip-portable-pack with both CMD ans PS shells, and it compiles and works.
However, the deprecated non-flutter build doesn't compile (it doesn't compile on the master branch either: NotADirectoryError: [WinError 267] The directory name is invalid: '../../resources') due to a problem with passing res_dir to the ./generate.py.

I'm planning to make a CI (or at least scripts) for building the Windows version natively on Windows or Windows containers, if this pull request gets merged. The reason is that, when you only need a Windows binary (e.g. developing on Win), it would be faster to compile only Windows-version.