Fix cargo install --index when used with registry.default

Author: arlosi

src/bin/cargo/commands/install.rs

@@ -126,10 +126,10 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
     } else if krates.is_empty() {
         from_cwd = true;
         SourceId::for_path(config.cwd())?
-    } else if let Some(registry) = args.registry(config)? {
-        SourceId::alt_registry(config, &registry)?
     } else if let Some(index) = args.get_one::<String>("index") {
         SourceId::for_registry(&index.into_url()?)?
+    } else if let Some(registry) = args.registry(config)? {
+        SourceId::alt_registry(config, &registry)?
     } else {
         SourceId::crates_io(config)?
     };