compiler: fix unused_peekable clippy lint

warning: `peek` never called on `Peekable` iterator
   --> compiler\rustc_session\src\utils.rs:130:13
    |
130 |     let mut args = std::env::args_os().map(|arg| arg.to_string_lossy().to_string()).peekable();
    |             ^^^^
    |
    = help: consider removing the call to `peekable`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_peekable

warning: `peek` never called on `Peekable` iterator
    --> compiler\rustc_trait_selection\src\traits\error_reporting\suggestions.rs:4934:17
     |
4934 |         let mut bounds = pred.bounds.iter().peekable();
     |                 ^^^^^^
     |
     = help: consider removing the call to `peekable`
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_peekable

compiler/rustc_session/src/utils.rs

@@ -127,7 +127,7 @@ pub fn extra_compiler_flags() -> Option<(Vec<String>, bool)> {
 
     const ICE_REPORT_COMPILER_FLAGS_STRIP_VALUE: &[&str] = &["incremental"];
 
-    let mut args = std::env::args_os().map(|arg| arg.to_string_lossy().to_string()).peekable();
+    let mut args = std::env::args_os().map(|arg| arg.to_string_lossy().to_string());
 
     let mut result = Vec::new();
     let mut excluded_cargo_defaults = false;