Fix i386/debian CI job and refactor cache key handling

Stan Hurequested to merge
sh-fix-i386-job into master

This required a number of changes:

  1. For rustc, we need to install --default-host=i686-unknown-linux-gnu otherwise the default x86_64 will get used.

  2. Drop full RubyGems update and only install bundler. Since we don't need to test the musl build, we can focus on installing a more recent version of bundler since gem update --system does not work in the i386/debian image.

  3. For simplicity, we can just use the CI_JOB_IMAGE as the cache key.

Edited by Stan Hu

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