Fix clippy suggestions

Leads to a performance improvement for `bat -A`:

    | Command | Mean [ms] | Min [ms] | Max [ms] | Relative |
    |:---|---:|---:|---:|---:|
    | `./bat-master --no-config -A ./bat-master` | 259.8 ± 1.1 | 258.4 | 261.7 | 1.15 ± 0.01 |
    | `./bat-2301 --no-config -A ./bat-master` | 225.6 ± 1.8 | 224.0 | 229.5 | 1.00 |

src/assets/build_assets/acknowledgements.rs

@@ -1,3 +1,4 @@
+use std::fmt::Write;
 use std::fs::read_to_string;
 use std::path::{Path, PathBuf};
 
@@ -124,7 +125,7 @@ fn append_to_acknowledgements(
     relative_path: &str,
     license_text: &str,
 ) {
-    acknowledgements.push_str(&format!("## {}\n\n{}", relative_path, license_text));
+    write!(acknowledgements, "## {}\n\n{}", relative_path, license_text).ok();
 
     // Make sure the last char is a newline to not mess up formatting later
     if acknowledgements