Honor system env vars as first option for compiling bicep when running on CI/CD

[vhvb1989]

input params are automatically mapped to ENV VARS when using Aspire. But, the primary strategy for azd to handle parameters is to use the environment configuration. Even if the environment variable which is mapped to an input parameter is set on CI env, azd tries to prompt if the value is not defined in the environment config. See: #4151

I talked to @ellismg in the past and agreed that, when running on CI, azd should check the system environment and respect the mapping before looking at environment-config.

This would enable folks to set unique env var secrets for each input, instead of using the AZD_INITIAL_ENVIRONMENT_CONFIG secret, holding all prompts values.