Windows: Fix arch detection via VCTOOLSINSTALLDIR if not first in PATH

[mhilbrunner]

Trying to invoke scons on Windows with VCTOOLSINSTALLDIR set (as is the case in the default VS / GDK command prompts) resulted in the following error for me:

KeyError: 'bin':
  ...
  File "C:\git\godot\.\platform\windows\detect.py", line 144:
    return msvc_target_aliases[first_path_arch]

Turns out we have this piece of code in the Windows platform detect.py, trying to determine the arch:

godot/platform/windows/detect.py

Lines 141 to 146 in 6b281c0

	# VS 2017 and newer.
	if os.getenv("VCTOOLSINSTALLDIR"):
	host_path_index = os.getenv("PATH").upper().find(os.getenv("VCTOOLSINSTALLDIR").upper() + "BIN\\HOST")
	if host_path_index > -1:
	first_path_arch = os.getenv("PATH").split(";")[0].rsplit("\\", 1)[-1].lower()
	return msvc_target_aliases[first_path_arch]

Here is what this tries to do:

• It checks if VCTOOLSINSTALLDIR is set
  (which for me has a value like C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.39.33519\).
• if so, it appends BIN\HOST to that value and searches for it in PATH and if found, saves the starting index of that in PATH.
• It should find a value like C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.39.33519\bin\Hostx64\x64 in PATH; it now tries to extract the arch bit in ...\Hostx64\... (x64).

However, there are 2 errors here:

1. Line 145 does not actually use the found index in the PATH. os.getenv("PATH").split(";")[0] just gets the first element of PATH, then .rsplit("\\", 1)[-1] cuts parts of. For me, this resulted in a value of bin instead of x64, as VCTOOLSINSTALLDIR was not the first in PATH. Changing it to os.getenv("PATH")[host_path_index:]... to actually start at the found index fixes this and resolves the error.
2. There is a missing guard to check whether the found arch actually exists in the dict before returning it. All other similar places in the file have such guards. So, I added one here with if first_path_arch in msvc_target_aliases.keys(), which would at least have prevented this error.

This seems to have been introduced in #64921.

[akien-mga]

Thanks!