EnvironmentScript.java

public class EnvironmentScript extends BuildWrapper {
	@SuppressWarnings("rawtypes")
	@Override
	public Environment setUp(AbstractBuild build,
			final Launcher launcher,
			final BuildListener listener) throws IOException, InterruptedException {

		// ... skip ...

		return new Environment() {
			@Override
			public void buildEnvVars(Map<String, String> env) {
				// Here we evaluate the variables we parsed above, in order.
				// We expand against all the environment variables we have so far.
				EnvVars vars = new EnvVars(env);
				
				listener.getLogger().println("Injecting env, before: " + env.toString());
				for (Map.Entry<String, String> variable : injectedVariables) {
					env.put(variable.getKey(), variable.getValue()); //vars.expand(variable.getValue()));
					listener.getLogger().println("Adding " + variable.toString());
				}
				listener.getLogger().println("Injecting env, after: " + env.toString());
			}
		};
	}
}

EnvironmentScriptTest.java

public class EnvironmentScriptTest extends HudsonTestCase {
	public void testWithDependentVariables () throws Exception {
		Project<?, ?> project = createFreeStyleProject();
		CaptureEnvironmentBuilder builder = new CaptureEnvironmentBuilder();
		project.getBuildersList().add(builder);
		project.getBuildWrappersList().add(new EnvironmentScript(SCRIPT_DEPENDENT_VARIABLES));

		Build<?, ?> b = assertBuildStatusSuccess(job.project.scheduleBuild2(0).get());
		System.out.println(b.getLog());

		EnvVars vars = builder.getEnvVars();
		assertEquals("one", vars.get("var1"));
		assertEquals("one two", vars.get("var2"));
		assertEquals("yo $var4", vars.get("var3"));
		assertEquals("three one two", vars.get("var4"));
	}
}

output

Injecting env, before: {..., USER=jtjerno, ...}
Adding var1=one
Adding var2=$var1 two
Adding var3=yo $var4
Adding var4=three ${var2}
Injecting env, after: {..., USER=jtjerno, var1=one, var2=$var1 two, var3=yo $var4, var4=three ${var2}, ...}