[ci/docker] Fix arm64 docker image builds by downgrading miniconda

[krfricke]

Why are these changes needed?

The latest release of miniconda ships with Python 3.8 as the base python. Trying to downgrade to Python 3.7 runs into unresolvable issues. Thus we pin miniconda until we stop building images for Python 3.7 (presumably once Python 3.7 is EOL, i.e. in ~2 months).

Related issue number

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• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[krfricke]