build-export: Add --arch option

This patch adds the --arch option to the build-export builtin command.

Previously build-export derives this from the metadata and then falls back
to xdg_app_get_arch(), except that this does not work when the metadata does
not specify an arch (such as with extensions like .Debug or .Locale).

https://bugs.freedesktop.org/show_bug.cgi?id=95226
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