Further split check-msrv steps; let Windows use pwsh again

This commit splits the commands in the steps of the `check-msrv`
job definition to be one per step, documenting each step.

In 510847b I predicted that it would not make sense to split the
steps to one command per step in `msrv.yml`. The change made there
of using `bash` even on Windows allowed for experimenting with how
the workflow logs look when reorganized in various ways. But in
view of other changes -- and to better clarify that two toolchains
were installed, but the MSRV toolchain is set default, so it is
used by all subsequent operations *except* the `-Zminimal-versions`
dependency downgrade that currently requires `nightly` -- it now
looks like having one command per step is better after all.

When running only one (simple) command per step, the main change in
510847b is no longer needed. That is, this now avoids that problem
in the same way it has been avoided in `test-fast` in `ci.yml`
since 4f2ab5b (#1559). So this commit also removes `shell: bash` in
`msrv.yml` (but keeps the comment clarification in `ci.yml`).

Author: EliahKagan

.github/workflows/msrv.yml

@@ -33,20 +33,16 @@ jobs:
       # IMPORTANT: adjust etc/msrv-badge.svg as well
       RUST_VERSION: 1.75.0
 
-    defaults:
-      run:
-        # Use `bash` even in the Windows job, so any failing command fails its step (due to `-e`).
-        shell: bash
-
     steps:
       - uses: actions/checkout@v4
       - uses: extractions/setup-just@v3
       - name: Set up ${{ env.RUST_VERSION }} (MSRV) and nightly toolchains
-        run: |
-          rustup toolchain install ${{ env.RUST_VERSION }} nightly --profile minimal --no-self-update
-          rustup default ${{ env.RUST_VERSION }}
+        run: rustup toolchain install ${{ env.RUST_VERSION }} nightly --profile minimal --no-self-update
+      - name: Set ${{ env.RUST_VERSION }} (MSRV) as default
+        run: rustup default ${{ env.RUST_VERSION }}
       - name: Downgrade locked dependencies to lowest allowed versions
         run: |
-          cargo +nightly update -Zminimal-versions  # TODO(msrv): Use non-`-Z` way when available.
+          # TODO(msrv): Use `cargo update --minimal-versions` when `--minimal-versions` is available.
+          cargo +nightly update -Zminimal-versions
       - name: Run some `cargo check` commands on `gix`
         run: just ci-check-msrv