Use bash on all platforms to try to fix Windows check-mode

Local testing shows that it does work on Windows, but on the GHA
Windows runner, it is not working. I'm not clear on the cause,
since it works via `just` when I test locally on Windows, but the
error that occurs on CI is due to `\` directory separators being
interpreted as escape characters and dropped:

    D:\a\gitoxide\gitoxide\target/debug/it check-mode
    /usr/bin/bash: line 1: D:agitoxidegitoxidetarget/debug/it: No such file or directory
    error: Recipe `check-mode` failed on line 262 with exit code 127

Since this is `justfile`-specific, it could be worked around if we
needed to run this routinely on Windows via CI, and as it is, we
don't need to do that, and the reason this is temporarily a matrix
job is just for a bit more testing beyond local testing.

Still, if possible it would be good to verify that this can work
on Windows on CI, and not just on my local Windows test machine.

Therefore, this uses `bash` on all platforms for the script step
that runs `just`, with the hope that this may help. On Windows GHA
runners, `shell: bash` uses Git Bash (rather than `pwsh`).

.github/workflows/ci.yml

@@ -366,6 +366,10 @@ jobs:
 
     runs-on: ${{ matrix.os }}
 
+    defaults:
+      run:
+        shell: bash
+
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable