ci: enable stricter Bash error checking (#2001)

Link: https://github.com/nix-community/stylix/pull/2001

Reviewed-by: Matt Sturgeon <matt@sturgeon.me.uk>
Reviewed-by: 0xda157 <da157@voidq.com>

.github/workflows/update-flake.yml

@@ -6,6 +6,10 @@ on:
     - cron: "0 0 1 * *"
   workflow_dispatch:
 
+defaults:
+  run:
+    shell: bash
+
 jobs:
   flake-update:
     runs-on: ubuntu-24.04