security: enforce HTTPS on curl downloads (SonarCloud S6506)

Add --proto '=https' to curl invocations so redirects cannot downgrade to
plaintext HTTP. Resolves SonarCloud S6506 (shell/docker/githubactions).

packaging/linux/package.sh

@@ -115,7 +115,7 @@ if has appimage; then
   if [ -z "$TOOL" ]; then
     TOOL="$CARGO_TARGET_DIR/appimagetool-$ARCH.AppImage"
     if [ ! -x "$TOOL" ]; then
-      curl -fsSL -o "$TOOL" "https://github.com/AppImage/appimagetool/releases/download/continuous/appimagetool-$ARCH.AppImage"
+      curl --proto '=https' -fsSL -o "$TOOL" "https://github.com/AppImage/appimagetool/releases/download/continuous/appimagetool-$ARCH.AppImage"
       chmod +x "$TOOL"
     fi
   fi