SCons : When building using MSys2/clang64 SCons thinks it is cross-compiling due to picking up env: $MINGW_PREFIX

### godot-cpp version

27ffd8c6be614b283b1228c3f8476db25070c00f

### System information

Windows 11, Msys2/clang64

### Issue description

Compiling for Windows on Windows using the MSys2/clang64 environment shell.
Using compile command: `scons verbose=yes use_mingw=yes use_llvm=yes`

SCons fails to archive the obj files using `C:/msys64/clang64/bin/x86_64-w64-mingw32-ar.exe` and supplies the following message:

```
C:/msys64/clang64/bin/x86_64-w64-mingw32-llvm-ar q
...
scons: *** [bin/libgodot-cpp.windows.template_debug.x86_64.a] The system cannot find the file specified
scons: building terminated because of errors.
```

The missing file is `C:/msys64/clang64/bin/x86_64-w64-mingw32-ar.exe`

The reason this is, is because for all MSys2 environments `$MINGW_PREFIX` is set to the root path of their subshell. And SCons picks up that environment variable as the default to `mingw_prefix`. When it comes to selecting the compiler, it chooses the cross compiling path. Except there is a misnomer here, all of these environments are for compiling native windows, there is no cross compiling. SCons then adds a bunch of architecture triplet information and tries to use the resultant binary.

Its a miracle that it works for ucrt64, or others, and if those prefixed binaries weren't by happenstance to be in the `$PATH` then it would fail.

The prefixed binaries: `C:\Msys2\$MINGW_PREFIX\bin\x86_64.w64.mingw32..exe` are there for some, as yet unknown to me, convenience. Since the environments are native windows, and the prefix is for the host system, they aren't cross compilers at all, and can be ignored entirely, al of the actually useful binaries have no prefix as this isnt a cross compiler.

The way I've come to understand MSys environments is that you choose the subshell for the type of Windows build you want to make, and or the libraries you want to link to. Cross compiling is another kettle of fish layered ontop of this.

[Msys Environments Table](https://www.msys2.org/docs/environments/)

Now, I'm not overly familiar with SCons, but I've been testing locally and here's what I have so far.

MSys environments conveniently define `$MSYSTEM` when one of the subshells are being used.

By using the `$MINGW_PREFIX` environment variable on the condition that `$MSYSTEM` is empty, I don't get the cross compile selection. And I can copy the `use_llvm` section from Linux to generate for clang.

```python
# windows.py:6
from SCons.Tool import mingw, msvc, clangxx, clang

# ...

# windows.py:76
def options(opts):
msystem = os.getenv("MSYSTEM", "")
mingw_prefix=""
if not msystem:
mingw_prefix = os.getenv("MINGW_PREFIX", "")

opts.Add("mingw_prefix", "MinGW prefix", mingw_prefix)

# ...

# windows.py:~133
elif (sys.platform == "win32" or sys.platform == "msys") and not env["mingw_prefix"]:
env["use_mingw"] = True
if env["use_llvm"]:
clang.generate(env)
clangxx.generate(env)
else:
mingw.generate(env)
```

I get a successful build, I haven't checked if any of the options are correct or what is missing tho.

AFAICT this also doesn't interfere with choosing a mingw_prefix for doing some actual cross compiling, but I am not yet upto testing that.

Cheers,
Samuel.

### Steps to reproduce

try to compile godot-cpp using MSys2 / clang64 environment.
Build Command: `scons verbose=yes use_mingw=yes use_llvm=yes`

### Minimal reproduction project

the godot-cpp source code. or test