Improvements prompted by clippy, and disable one lint

Improvements prompted by current nightly's clippy.

Also disable one lint which I think is in rather poor taste.

crates/arti/src/lib.rs

@@ -122,6 +122,8 @@ pub mod process;
 pub mod socks;
 pub mod watch_cfg;
 
+use std::fmt::Write;
+
 pub use cfg::{
     ApplicationConfig, ApplicationConfigBuilder, ArtiConfig, ArtiConfigBuilder, ProxyConfig,
     ProxyConfigBuilder, SystemConfig, SystemConfigBuilder,
@@ -231,7 +233,7 @@ pub fn main_main() -> Result<()> {
     // explicitly or not.
     let mut config_file_help = "Specify which config file(s) to read.".to_string();
     if let Some(default) = arti_config::default_config_file() {
-        config_file_help.push_str(&format!(" Defaults to {:?}", default));
+        write!(config_file_help, " Defaults to {:?}", default).unwrap();
     }
 
     let matches =