Environment variable MSI_ENDPOINT and IDENTITY_HEADER should be available to pre-build environment

[lb-consult]

We are using a "PRE_BUILD_COMMAND" to customise the build environment for our Azure Function (Python on Linux)

The script need to access some external resources protected using EntraID-based authentication. It would be helpful if we could use the managed-identity in this process. To do so the pre-build script would need access to the environment variables MSI_ENDPOINT and IDENTITY_HEADER.