ci: Execute rest_api_test tests in codeconv

The tests in the rest_api_test.go file require dbus-launcher,
available in dbus-x11. If that binary isn't available, the tests
are skipped.

A clear proof of this is the Codeconv checks, which show that
the code in rest_api.go isn't being checked, even when there
are tests that cover it.

This patch fixes this.

Author: sergio-costas

.github/actions/download-install-debian-deps/action.yaml

@@ -27,6 +27,6 @@ runs:
         sudo apt update
         ln -s packaging/ubuntu-16.04 debian
         sudo apt build-dep -y "${{ inputs.snapd-src-dir }}"
-        sudo apt install -y clang
+        sudo apt install -y clang dbus-x11
         sudo apt install -y gcovr lcov
         rm -rf ./debian-deps