Resolve uninlined_format_args pedantic clippy lint in build script

    warning: variables can be used directly in the `format!` string
       --> build.rs:140:9
        |
    140 | /         eprintln!(
    141 | |             "Environment variable ${} is not set during execution of build script",
    142 | |             key,
    143 | |         );
        | |_________^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
        = note: `-W clippy::uninlined-format-args` implied by `-W clippy::pedantic`
        = help: to override `-W clippy::pedantic` add `#[allow(clippy::uninlined_format_args)]`

build.rs

@@ -137,10 +137,7 @@ fn compile_probe(rustc_bootstrap: bool) -> bool {
 
 fn cargo_env_var(key: &str) -> OsString {
     env::var_os(key).unwrap_or_else(|| {
-        eprintln!(
-            "Environment variable ${} is not set during execution of build script",
-            key,
-        );
+        eprintln!("Environment variable ${key} is not set during execution of build script");
         process::exit(1);
     })
 }