fix: extract tkinter from Python installer via 7z, fix NSIS icon path

- Replace nuget approach (doesn't contain tkinter) with extracting
  from the official Python amd64.exe installer using 7z
- Remove MUI_ICON/MUI_UNICON (no .ico file available, use NSIS default)
- Add p7zip-full to CI dependencies

.gitea/workflows/release.yml

@@ -63,7 +63,7 @@ jobs:
       - name: Install system dependencies
         run: |
           sudo apt-get update
-          sudo apt-get install -y --no-install-recommends zip libportaudio2 nsis
+          sudo apt-get install -y --no-install-recommends zip libportaudio2 nsis p7zip-full
 
       - name: Cross-build Windows distribution
         run: |