linux/appimage: install meson and ninja from PyPI

Some systems (e.g. Ubuntu 22.04, which we use to make the AppImage),
have ancient versions of meson and ninja, which is not good when we
have dependencies like PyGObject 3.47+ (and more recently, one of its
transitive dependencies, pycairo 1.27+) requiring a fairly recent
version of meson to be able to be built.

So, instead of relying on whatever meson/ninja version the current
system provides us with to make the AppImage, this change will simply
install up-to-date, official wheels of meson/ninja directly from PyPI.

This also allows us to finally unpin PyGObject's version.

appimage/AppImageBuilder.yml

@@ -22,6 +22,8 @@ script:
   - cp -r "$SOURCE_DIR/../lang" "$SOURCE_DIR/../icons" "$SOURCE_DIR"/../*.py "$TARGET_APPDIR/usr/src"
   - cp "$SOURCE_DIR/pickaxe.png" "$TARGET_APPDIR/usr/share/icons/hicolor/128x128/apps/io.github.devilxd.twitchdropsminer.png"
 
+  # Create a virtual environment and install up-to-date versions of meson and ninja.
+  - python3 -m venv env && source ./env/bin/activate && python3 -m pip install meson ninja
   # Install requirements.
   - python3 -m pip install --ignore-installed --prefix=/usr --root="$TARGET_APPDIR" -r "$SOURCE_DIR/../requirements.txt" certifi
   # Generate byte-code files beforehand, for slightly faster app startup.