twoliter make: take go modules from environment

In #130 1410d5b we startred requiring GO_MODULES to come from the
environment when using twoliter make. This appeared to work because go
mod vendor had been cached when testing. But in fact, we were not taking
the environment variable at all! So this commit fixes it.

twoliter/src/cargo_make.rs

@@ -160,14 +160,15 @@ fn build_system_env_vars() -> Result<Vec<String>> {
 
 /// A list of environment variables that don't conform to naming conventions but need to be passed
 /// through to the `cargo make` invocation.
-const ENV_VARS: [&str; 12] = [
+const ENV_VARS: [&str; 13] = [
     "ALLOW_MISSING_KEY",
     "AMI_DATA_FILE_SUFFIX",
     "CARGO_MAKE_CARGO_ARGS",
     "CARGO_MAKE_CARGO_LIMIT_JOBS",
     "CARGO_MAKE_DEFAULT_TESTSYS_KUBECONFIG_PATH",
     "CARGO_MAKE_TESTSYS_ARGS",
     "CARGO_MAKE_TESTSYS_KUBECONFIG_ARG",
+    "GO_MODULES",
     "MARK_OVA_AS_TEMPLATE",
     "RELEASE_START_TIME",
     "SSM_DATA_FILE_SUFFIX",
@@ -214,6 +215,7 @@ fn test_is_build_system_env() {
     assert!(is_build_system_env("TESTSYS_!"));
     assert!(is_build_system_env("BOOT_CONFIG!"));
     assert!(is_build_system_env("BOOT_CONFIG_INPUT"));
+    assert!(is_build_system_env("GO_MODULES"));
     assert!(is_build_system_env("AWS_REGION"));
     assert!(!is_build_system_env("PATH"));
     assert!(!is_build_system_env("HOME"));