run mage buildAll || true when building the plugin (#4658)

# What this PR does

`oncall-private` is using
[actions/build-sign-and-package-plugin/action.yml](https://github.com/grafana/oncall/pull/4658/files#diff-6200ac63c387869ca55bc34d1b343a96c667daef428602f910d696836cb09f84)
from the dev branch of this repo when doing the deployment. In order to
deploy new OnCall initialization which has Go backend component, we need
this action to install Go, Mage and do the `mage buildAll`. For other
branches where there are no Go files yet, it would just swallow the
error thanks to the `|| true` part

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## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

.github/actions/build-sign-and-package-plugin/action.yml

@@ -27,6 +27,13 @@ runs:
       # yamllint disable rule:line-length
       run: |
         echo filename="grafana-oncall${{ inputs.is_enterprise == 'true' && '-ee' || '' }}-app-${{ inputs.plugin_version_number }}.zip" >> $GITHUB_OUTPUT
+    - name: Install Go
+      uses: actions/setup-go@v4
+      with:
+        go-version: "1.21.5"
+    - name: Install Mage
+      shell: bash
+      run: go install github.com/magefile/mage@v1.15.0
     - name: Build, sign, and package plugin
       shell: bash
       working-directory: ${{ inputs.working_directory }}
@@ -35,6 +42,7 @@ runs:
       run: |
         jq --arg v "${{ inputs.plugin_version_number }}" '.version=$v' package.json > package.new && mv package.new package.json && jq '.version' package.json;
         yarn build
+        mage buildAll || true
         yarn sign
         if [ ! -f dist/MANIFEST.txt ]; then echo "Sign failed, MANIFEST.txt not created, aborting." && exit 1; fi
         mv dist grafana-oncall-app