build: Use distinct gettext domains when exporting zips

Since commit a6ee142f, the extension archives that are uploaded to extensions.gnome.org only contain strings that are relevant for the extension, not all translations from all extensions.

Unfortunately all extensions still share a common gettext domain, so the extension with the last bind_textdomain() call wins and leaves the others without translations.

Address this by using distinct domains when not installed system-wide.

Fixes #335 (closed)