chore: setup buildx on test too and --load (#114)

## Description

This tests buildx setup on test and adds `--load`

## Related Issue

Fixes #N/A


## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] 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/uds-package-mattermost/blob/main/CONTRIBUTING.md#developer-workflow)
followed

.github/workflows/test.yaml

@@ -52,6 +52,8 @@ jobs:
           registry1Password: ${{ secrets.IRON_BANK_ROBOT_PASSWORD }}
           ghToken: ${{ secrets.GITHUB_TOKEN }}
 
+      - uses: docker/setup-buildx-action@aa33708b10e362ff993539393ff100fa93ed6a27 # v3.5.0
+
       - name: Test
         uses: defenseunicorns/uds-common/.github/actions/test@91515ef86914334356c35ffb4b2c2c5fb4d19174 # v0.9.0
         with:

plugins/zarf.yaml

@@ -18,4 +18,4 @@ components:
           # we always build on amd64 because that is the target architecture for the plugin binaries
           # but we still need fake arm64 images because @zachariahmiller said so
           - cmd: |
-              docker buildx build . -t uds-package-mattermost/mattermost-extra-plugins:latest --platform linux/amd64
+              docker buildx build --load . -t uds-package-mattermost/mattermost-extra-plugins:latest --platform linux/amd64