Standardize the name used for azd environment variables

Make `azd` to follow one only convention for environment variables.
`azd` should use a prefix to avoid any collision with system variables.

azd is currently using environment variables in the form of:
- `AZD_XXXX_XXXX...` , like:
- AZD_SKIP_UPDATE_CHECK
- AZD_DEBUG_FORCE_NO_TTY
- AZD_FUNC_TEST

- `AZURE_DEV_XXXX_XXX...`, like:
- AZURE_DEV_COLLECT_TELEMETRY
- AZURE_DEV_CI_OS
- AZURE_DEV_USER_AGENT

- `AZURE_XXX...`, like:
- AZURE_ENV_NAME

- `XXX...`, like: (this should be fixed as could potentially collision)
- GITHUB_ACTIONS
- NO_COLOR
- CODESPACES
- REMOTE_CONTAINERS

Extra note: `azd` should also define if setting env vars with empty value is equivalent to adding any value to it.
For example, for bool vars like `NO_COLOR`, is setting `NO_COLOR=` the same as `NO_COLOR=1`? or is the same as not setting the env var.