podcasts-gtk: make it possible to compile with just cargo

This is not supported at all, and it still won't run with cargo
run, thankfully, you should use meson instead...

The only purpose of this commit is to make it possible for
cargo check, and by extension rls, to function and work
properly.

Part of #110

scripts/test.sh

@@ -23,9 +23,6 @@ flatpak-builder --run \
 # Run the tests
 xvfb-run -a -s "-screen 0 1024x768x24" \
     flatpak-builder --run \
-    --env=APP_ID="org.gnome.PodcastsDevel" \
-    --env=LOCALEDIR="./podcasts-gtk/po" \
-    --env=VERSION="0.0.0" \
     --env=CARGO_HOME="target/cargo-home" \
     --env=CARGO_TARGET_DIR="target_test/" \
     app ${MANIFEST_PATH} \