When running in a pipeline azd provision always sets the main container app to use the hello world image

[CMeeg]

Description of issue / Reproduction steps

• The fix for Running azd provision after azd deploy sets the main container app to use the hello world image #5 worked when running locally, but not when running in a pipeline
• When running in a pipeline the SERVICE_WEB_IMAGE_NAME env var is not available so cannot be provided to the bicep deployment

Expected behaviour

• When running in a pipeline azd provision should not change the image deployed to the main Container app

Actual behaviour

• When running in a pipeline azd provision changes the image deployed to the main Container app to mcr.microsoft.com/azuredocs/containerapps-helloworld:latest