fix(pipelines): var group conditional must use compile time syntax

[julie-ng]

Not 100% sure why this doesn't work

variables:
- template: vars/global.yaml
- ${{ if eq(variables.isMain, 'True') }}:
  - group: e2e-gov-demo-dev-kv
- ${{ if eq(variables.isProduction, 'True') }}:
  - group: e2e-gov-demo-kv

but my guess is that variables.isMain etc. isn't immediately available because they are defined in vars/global.yaml and may not be able at compile time in the next line of code. Switching back to variables['Build.SourceBranch'] which is a predefined system variable and available from beginning.

So hoping this syntax works:

- ${{ if eq(variables['Build.SourceBranch'], 'refs/heads/main') }}:
  - group: e2e-gov-demo-dev-kv

[azure-pipelines]

🟢  No configuration drift detected

Source Version: ad22a76