CI: set CARGO_HOME before cargo test

This is set in the scripts/cargo.sh during building, but since we
invoke cargo test manually we need to also set it here.

.gitlab-ci.yml

@@ -22,8 +22,15 @@ flatpak:
         - flatpak build-export repo app
 
         # Run the tests
-        - flatpak-builder --run --env=RUSTFLAGS="--cfg rayon_unstable" app org.gnome.Podcasts.json cargo test -- --test-threads=1
+        - |
+          flatpak-builder --run \
+              --env=CARGO_HOME="./target/cargo-home/" \
+              --env=RUSTFLAGS="--cfg rayon_unstable" \
+              app org.gnome.Podcasts.json \
+              cargo test -- --test-threads=1
+
         # - flatpak-builder --run app org.gnome.Podcasts.json cargo test -- --test-threads=1 --ignored
+
         # Create a flatpak bundle
         - flatpak build-bundle repo ${BUNDLE} org.gnome.Podcasts
 
@@ -76,4 +83,4 @@ rustfmt:
     script:
         - rustc -Vv && cargo -Vv
         - cargo fmt --version
-        - cargo fmt --all -- --check
+        - cargo fmt --all -- --check