Clarify the various purposes of shell: bash

We set `shell: bash` explicitly in several places, for three
different reasons. At least two of the reasons (those in `ci.yml`)
are not not obvious without explanation. This clarifies the
existing comments that explained this, and adds such comments
where absent.

Author: EliahKagan

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
           )
           apt-get update
           apt-get install --no-install-recommends -y -- "${prerequisites[@]}"
-        shell: bash
+        shell: bash  # This step needs `bash`, and the default in container jobs is `sh`.
       - name: Verify that we are in an environment with limited dev tools
         run: |
           set -x
@@ -264,7 +264,7 @@ jobs:
           dpkg --add-architecture ${{ matrix.runner-arch }}
           apt-get update
           apt-get install --no-install-recommends -y -- "${prerequisites[@]}"
-        shell: bash
+        shell: bash  # This step needs `bash`, and the default in container jobs is `sh`.
       - uses: actions/checkout@v4
       - name: Install Rust via Rustup
         run: |
@@ -427,7 +427,7 @@ jobs:
 
     defaults:
       run:
-        shell: bash  # Without specifying this, we don't get `-o pipefail`.
+        shell: bash  # Without this, the shell here is `bash` but without `-o pipefail`.
 
     steps:
       - name: Find this workflow

.github/workflows/release.yml

@@ -17,7 +17,7 @@ permissions:
 
 defaults:
   run:
-    shell: bash
+    shell: bash  # Use `bash` even in the Windows jobs.
 
 jobs:
   # Create a draft release, initially with no binary assets attached.