CI: fix find_libpython invocation for Python DLL resolution

main.yml resolved PYTHONNET_PYDLL via "python -m find_libpython", but this
fork vendors the module as pythonnet.find_libpython. The old top-level
invocation failed with "No module named find_libpython", leaving
PYTHONNET_PYDLL empty and crashing every embedding test in
PythonEngine.Initialize() with DllNotFoundException ("Could not load .").

Use "python -m pythonnet.find_libpython" in both the Windows and non-Windows
env-setup steps, matching ARM.yml and nuget-preview.yml.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jhonabreul

.github/workflows/main.yml

@@ -54,13 +54,13 @@ jobs:
       - name: Set Python DLL path and PYTHONHOME (non Windows)
         if: ${{ matrix.os != 'windows' }}
         run: |
-          echo PYTHONNET_PYDLL=$(python -m find_libpython) >> $GITHUB_ENV
+          echo PYTHONNET_PYDLL=$(python -m pythonnet.find_libpython) >> $GITHUB_ENV
           echo PYTHONHOME=$(python -c 'import sys; print(sys.prefix)') >> $GITHUB_ENV
 
       - name: Set Python DLL path and PYTHONHOME (Windows)
         if: ${{ matrix.os == 'windows' }}
         run: |
-          Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append -InputObject "PYTHONNET_PYDLL=$(python -m find_libpython)"
+          Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append -InputObject "PYTHONNET_PYDLL=$(python -m pythonnet.find_libpython)"
           Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append -InputObject "PYTHONHOME=$(python -c 'import sys; print(sys.prefix)')"
 
       - name: Embedding tests