Do not mark continuous as "prerelease" (AppImage#95)

This has led to confusion, as people thought there is "no recent release". In fact, continuous is always the only supported release.

Author: probonopd

.github/workflows/build.yaml

@@ -116,4 +116,4 @@ jobs:
         run: |
             wget -q https://github.com/TheAssassin/pyuploadtool/releases/download/continuous/pyuploadtool-x86_64.AppImage
             chmod +x pyuploadtool-x86_64.AppImage
-            ./pyuploadtool-x86_64.AppImage --appimage-extract-and-run artifacts*/*
+            GITHUB_CONTINUOUS_RELEASE_TYPE=stable ./pyuploadtool-x86_64.AppImage --appimage-extract-and-run artifacts*/*