Don't run the big bang test for externals who don't have the IB usern…

…ame (#1432)

## Description

This skips running the big bang test if the Iron Bank secret is not
present

## Related Issue

Fixes #N/A

## Type of change

- [X] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Other (security config, docs update, etc)

## Checklist before merging

- [X] Test, docs, adr added or updated as needed
- [X] [Contributor Guide
Steps](https://github.com/defenseunicorns/zarf/blob/main/CONTRIBUTING.md#developer-workflow)
followed

.github/workflows/test-bigbang.yml

@@ -45,7 +45,7 @@ jobs:
           password: ${{ secrets.IRONBANK_PASSWORD }}
 
       - name: Run tests
-        run: go test ./src/extensions/bigbang/test -failfast -v -timeout 30m
+        run: "[ -n \"${{ secrets.IRONBANK_USERNAME }}\" ] && go test ./src/extensions/bigbang/test -failfast -v -timeout 30m"
 
       - name: Save logs
         if: always()