ci: Set `shell: bash` as a default, remove duplicates
A follow-up to #74406, this commit merely removes the `shell: bash` lines where they were added in favor of setting defaults for *all* "run" steps in the jobs that run the tests.
The changes in #74406 were needed because of an upstream change to the `windows-2019` GitHub Actions image. Previously, the configuration worked fine without specifying `shell: bash`, but for some reason this broke with a new change that was deployed today. The preceding PR was a hotfix to get CI passing, but there was a slightly less duplicative way to specify the default shell for the jobs, which was to set the `defaults.run` option.
This change applies to the `pr`, `try`, `auto`, and `auto-fallible` jobs, which are derived from the YAML-anchor `base-ci-job`. I did not apply these changes to the `master`, `try-success`, `try-failure`, `auto-success`, or `auto-failure` jobs because they have only a few steps.
cc/r? @Mark-Simulacrum
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