With the releases of Composer 2.2.2 and 2.2.3 (and PHPUnit 9.5.11), running the tests with Composer 2 in the pipeline should be safe afain.
Composer versions 2.2.0 and 2.2.1 have a bug that breaks PHPUnit. Until version 2.2.2 is released with a fix, we need to keep to Composer 2.1 to keep the tests from breaking. See https://github.com/composer/composer/issues/10387 for details.
Also stop running the other CI workflow on every push/PR (as the results of both workflows provide the same results). Fixes #275