clippy-driver: if --sysroot is specified on the command line, use that

If the user explicitly sets sysroot on the command line, then use that
value.

Issue #3663

src/driver.rs

@@ -72,8 +72,11 @@ pub fn main() {
                 exit(0);
             }
 
+            let mut orig_args: Vec<String> = env::args().collect();
+
             let sys_root = option_env!("SYSROOT")
                 .map(String::from)
+                .or_else(|| arg_value(&orig_args, "--sysroot", |_| true).map(|s| s.to_string()))
                 .or_else(|| std::env::var("SYSROOT").ok())
                 .or_else(|| {
                     let home = option_env!("RUSTUP_HOME").or(option_env!("MULTIRUST_HOME"));
@@ -93,7 +96,6 @@ pub fn main() {
 
             // Setting RUSTC_WRAPPER causes Cargo to pass 'rustc' as the first argument.
             // We're invoking the compiler programmatically, so we ignore this/
-            let mut orig_args: Vec<String> = env::args().collect();
             if orig_args.len() <= 1 {
                 std::process::exit(1);
             }