Revert default_missing_value to default_value (#380)

We don't need to do this anymore. It was some change I was testing that
didn't work, and I forgot to remove it.

src/settings.rs

@@ -221,7 +221,7 @@ pub struct CommonSettings {
         global = true,
         value_parser = crate::diagnostics::diagnostic_endpoint_validator,
         num_args = 0..=1, // Required to allow `--diagnostic-endpoint` or `NIX_INSTALLER_DIAGNOSTIC_ENDPOINT=""`
-        default_missing_value = "https://install.determinate.systems/nix/diagnostic"
+        default_value = "https://install.determinate.systems/nix/diagnostic"
     )]
     pub diagnostic_endpoint: Option<String>,
 }