Replace os.Setenv with t.Setenv in all unit tests

[antoninbas]

We have a few unit tests which require environment variables to be set. At the moment, we typically call os.Setenv, with a deferred call to os.Unsetenv to undo the change. For example:

antrea/pkg/util/env/env_test.go

Lines 42 to 43 in bf06790

	_ = os.Setenv(NodeNameEnvKey, k)
	defer os.Unsetenv(NodeNameEnvKey)

https://pkg.go.dev/testing#T.Setenv has been available since Go 1.17. It is a cleaner way to set environment variables for unit tests, and the variable will be unset automatically during test cleanup. We already use it in one test file: https://github.com/antrea-io/antrea/blob/main/pkg/apiserver/certificate/selfsignedcert_provider_test.go

All existing unit tests should be updated to use t.Setenv instead of os.Setenv, for readability and consistency.