github: Don't add the bundled Python to PATH in test-llvm

Previously, we added the llvm-mingw bundled python to the head
of PATH, but we didn't expect to use it after all.

(In other tests, where we explicitly want to use the llvm-mingw
bundled python, we set -DPython3_EXECUTABLE="$Env:PYTHON_EXE.)

Previously (with runner image version 20260622.87.1), CMake ended
up choosing
C:/hostedtoolcache/windows/Python/3.13.14/x64/python3.exe, but
now (with runner image version 20260629.93.2) we ended up picking
C:/llvm-mingw/python/bin/python3.14.exe instead.

With the newer runner images, we do have
C:\hostedtoolcache\windows\Python\3.13.14\arm64\python3.exe in
PATH, but it is unclear why that doesn't get picked.

(Picking another version than the first one in PATH would make
sense if we'd have Python_FIND_STRATEGY=VERSION set. According to
the docs, that shouldn't be the default any longer though, and it
wouldn't explain why it picked 3.13 before when we provide
3.14.)

In any case, just skip adding our version to PATH since we
don't intend to use it, to make it use the system provided
version.

.github/workflows/test-llvm.yml

@@ -62,8 +62,6 @@ jobs:
           del llvm-mingw-*.zip
           mv llvm-mingw-* c:\llvm-mingw
           echo "c:\llvm-mingw\bin" | Out-File -FilePath $Env:GITHUB_PATH -Encoding utf8 -Append
-          echo "c:\llvm-mingw\python\bin" | Out-File -FilePath $Env:GITHUB_PATH -Encoding utf8 -Append
-          echo "PYTHON_EXE=c:/llvm-mingw/python/bin/python3.exe" | Out-File -FilePath $Env:GITHUB_ENV -Encoding utf8 -Append
       - uses: actions/checkout@v6
       - name: Checkout llvm-project
         env: