I don't think environment variables are available by default in Tomcat configuration files. System properties, however, are. So if you add your variable as a system property instead of an environment variable, it should work.
If you for some reason absolutely have to use environment variables, the Tomcat documentation has a reference to Property replacements in it's section about System Properties that can be set to customize Tomcat behaviour:
org.apache.tomcat.util.digester. PROPERTY_SOURCE
Set this to a fully qualified name of a class that implements org.apache.tomcat.util.IntrospectionUtils.PropertySource. Required to have a public constructor with no arguments.
Use this to add a property source, that will be invoked when ${parameter} denoted parameters are found in the XML files that Tomcat parses.