ci: Replace exec-with-shell wrapper with "plain bash"

Also, promote defaults.run.shell from inside only the primary jobs to
the top level.

The src/ci/exec-with-shell.py wrapper script was formerly used to change
out the shell mid-job by intercepting a CI_OVERRIDE_SHELL environment
variable.  Now, instead, we just set `bash` as the global default across
all jobs, and we also delete the exec-with-shell.py script.

Signed-off-by: Kristofer Rye <kristofer.rye@gmail.com>

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ name: CI
       - "**"
 defaults:
   run:
-    shell: "python src/ci/exec-with-shell.py {0}"
+    shell: bash
 jobs:
   pr:
     name: PR
@@ -51,9 +51,6 @@ jobs:
             os: ubuntu-latest-xl
     timeout-minutes: 600
     runs-on: "${{ matrix.os }}"
-    defaults:
-      run:
-        shell: bash
     steps:
       - name: disable git crlf conversion
         run: git config --global core.autocrlf false
@@ -153,9 +150,6 @@ jobs:
             env: {}
     timeout-minutes: 600
     runs-on: "${{ matrix.os }}"
-    defaults:
-      run:
-        shell: bash
     steps:
       - name: disable git crlf conversion
         run: git config --global core.autocrlf false
@@ -473,9 +467,6 @@ jobs:
             os: windows-latest-xl
     timeout-minutes: 600
     runs-on: "${{ matrix.os }}"
-    defaults:
-      run:
-        shell: bash
     steps:
       - name: disable git crlf conversion
         run: git config --global core.autocrlf false
@@ -601,9 +592,6 @@ jobs:
             os: macos-latest
     timeout-minutes: 600
     runs-on: "${{ matrix.os }}"
-    defaults:
-      run:
-        shell: bash
     steps:
       - name: disable git crlf conversion
         run: git config --global core.autocrlf false

src/ci/github-actions/ci.yml

@@ -75,9 +75,6 @@ x--expand-yaml-anchors--remove:
   - &base-ci-job
     timeout-minutes: 600
     runs-on: "${{ matrix.os }}"
-    defaults:
-      run:
-        shell: bash
     env: *shared-ci-variables
     steps:
       - name: disable git crlf conversion
@@ -233,18 +230,10 @@ on:
 
 defaults:
   run:
-    # While on Linux and macOS builders it just forwards the arguments to the
-    # system bash, this wrapper allows switching from the host's bash.exe to
-    # the one we install along with MSYS2 mid-build on Windows.
-    #
-    # Once the step to install MSYS2 is executed, the CI_OVERRIDE_SHELL
-    # environment variable is set pointing to our MSYS2's bash.exe. From that
-    # moment the host's bash.exe will not be called anymore.
-    #
-    # This is needed because we can't launch our own bash.exe from the host
-    # bash.exe, as that would load two different cygwin1.dll in memory, causing
-    # "cygwin heap mismatch" errors.
-    shell: python src/ci/exec-with-shell.py {0}
+    # On Linux, macOS, and Windows, use the system-provided bash as the default
+    # shell. (This should only make a difference on Windows, where the default
+    # shell is PowerShell.)
+    shell: bash
 
 jobs:
   pr: