testing: setting env var incompatible with parallel tests (#14405)

Neither the `os.Setenv` nor `t.Setenv` helper are safe to use in parallel tests
because environment variables are process-global. The stdlib panics if you try
to do this. Remove the `ci.Parallel()` call from all tests where we're setting
environment variables.

command/ui_test.go

@@ -5,13 +5,11 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/hashicorp/nomad/ci"
 	"github.com/mitchellh/cli"
 	"github.com/stretchr/testify/require"
 )
 
 func TestCommand_Ui(t *testing.T) {
-	ci.Parallel(t)
 
 	type testCaseSetupFn func(*testing.T)