ci: Skip Tar interop tests in main unit test job

[euanh]

Motivation

The main unit test job needs to match the pull_request unit test job. Both jobs depend on swift-nio's shared unit test job, so it's unclear whether this can be abstracted further.

Modifications

• Update the --skip parameters in main.yml to match pull_request.yml

Result

The results of main unit test job should match those of the pull_request unit test job.

Test Plan

Unit tests run on the main branch should pass again.