…r=Mark-Simulacrum

Set shell for github actions CI

r? @pietroalbini but because this seems at worst harmless and CI is broken self approving

…roalbini

ci: Set `shell: bash` as a default, remove duplicates

A follow-up to rust-lang#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 rust-lang#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