Make the config collation independent of a specified config file

As there are several potential sources of configuration, the collation
mode is relevant whether a invocation-specific configuration file is
specified, or not. This has the cheery consequence of sidestepping
the issue described in clap-rs/clap#5020.

topiary-cli/src/cli.rs

@@ -46,14 +46,7 @@ pub struct GlobalArgs {
         display_order = 101,
         env = "TOPIARY_CONFIG_COLLATION",
         global = true,
-        hide_env_values = true,
-
-        // FIXME There appears to be a bug with clap: If this argument is specified via its
-        // environment variable, then the required argument (--configuration) *only* works if it is
-        // also specified via its environment variable. If you use the CLI argument, it complains
-        // that the argument doesn't exist. This behaviour only occurs with subcommands, but that
-        // is exactly our use case, here. (See clap-rs/clap#5020)
-        requires = "configuration"
+        hide_env_values = true
     )]
     pub configuration_collation: Option<configuration::CollationMode>,
 }