Triggered build parameters are loaded at the infrastructure part of the lifecycle. Environment variables are loaded after this, in the machine part of the lifecycle. As a result, it is not currently possible to override existing environment variable values via build parameters. At least for us, it is desirable to not only create new environment variables but also override their default values as well.
Here is an example of the issue: https://circleci.com/gh/cloud-elements/hykes-provisioner/101 The default TEST_INTEGRATION value is 1 and I am attempting to override it to 0 via the trigger API.