pass apiClient to determineBaseRepo

Our code had an unspoken assumption that only one apiClient is created
during the course of a command. Violating this assumption is fine in
almost all cases, but not when we need to do a re-auth to add a new
oauth scope to a user's token.

There is likely a more elegant solution to the problem but until then
this changes determineBaseRepo to use an existing apiClient.

command/pr_checkout.go

@@ -34,7 +34,7 @@ func prCheckout(cmd *cobra.Command, args []string) error {
 	}
 
 	if baseRepo == nil {
-		baseRepo, err = determineBaseRepo(cmd, ctx)
+		baseRepo, err = determineBaseRepo(apiClient, cmd, ctx)
 		if err != nil {
 			return err
 		}