google-cloud-sdk: Fix default component arches
When a google-cloud-sdk component does not specify architectures
on components.json we should fallback to all supported arches.
Previously there was a bug where `allArches` were incorrectly
taken from the `attrValues arches` (nixOS architectures) instead of
the GCS ones using `attrNames arches`.
Also added arm to the list of supported architectures. No changes
needed on components.json since it looks urls and hashes were already
there.
Co-authored-by: Fabián Heredia Montiel <fabianhjr@protonmail.com>