chore: configure SHELL env in CI jobs

Maybe fix problem with erlexec failing to start:

```
SHELL environment variable not set! [c_src/exec.cpp:609]

Notice: 15:48:19.772 [notice] Application erlexec exited: :exec_app.start(:normal, []) returned an error: shutdown: failed to start child: :exec
    ** (EXIT) {:port_exited_with_status, 4}
```

.github/workflows/ci.yml

@@ -18,6 +18,7 @@ jobs:
       STRIPE_MOCK_VERSION: 0.144.0
       STRIPE_SECRET_KEY: non_empty_string
       SKIP_STRIPE_MOCK_RUN: true
+      SHELL: bash
     runs-on: ubuntu-20.04
     name: Test (OTP ${{matrix.otp}} / Elixir ${{matrix.elixir}})
     strategy: