Fix i386/debian CI job and refactor cache key handling
This required a number of changes:
-
For rustc, we need to install
--default-host=i686-unknown-linux-gnu
otherwise the defaultx86_64
will get used. -
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. -
For simplicity, we can just use the CI_JOB_IMAGE as the cache key.
Edited by Stan Hu