Adapt to new return-type of IExecutionEnvironmentsManager.getSupportedExecutionEnvironments()

[HannesWell]

What it does

Adapt to new return-type of IExecutionEnvironmentsManager.getSupportedExecutionEnvironments(), changed in https://github.com/eclipse-jdt/eclipse.jdt.debug/pull/474.

~Without that change being submitted, this PR's build cannot succeed (and without this, an I-Build with the other PR applied cannot succeed too).~ Without that change this PR leads to wrong display orders of the EEs.

Author checklist

• [ ] I have thoroughly tested my changes
• [x] The change is following the coding conventions
• [x] I have signed the Eclipse Contributor Agreement (ECA)