Clarify usage of env.ENV_VAR instead of $ENV_VAR in when conditions

[dam-ease]

When using a condition within the when block of YAML script, referencing environment variables with $ENV_VAR, as done in other parts of the YAML script, results in an error:

When condition '$SHOREBIRD_ENABLED == "true"' is invalid: 'unexpected char '$' at 0
line 1'

The right approach is to use env.ENV_VAR instead of $ENV_VAR when referencing the environment variables. This distinction is not explicitly stated in the documentation. Although env.ENV_VAR is used in the example code snippet provided in the documentation, there is no clear explanation or note highlighting this requirement. https://docs.codemagic.io/yaml-running-builds/starting-builds-automatically/#using-condition-inside-when.

Updating the docs to state the requirement to use env.ENV_VAR in when conditions would give more clarity.