Avoid using env::temp when linking a binary

This keeps all build artefacts (even temporary ones) within the build directory.

compiler/rustc_fs_util/src/lib.rs

@@ -1,6 +1,6 @@
 use std::ffi::{CString, OsStr};
 use std::path::{Path, PathBuf, absolute};
-use std::{env, fs, io};
+use std::{fs, io};
 
 use tempfile::TempDir;
 
@@ -139,8 +139,4 @@ impl<'a, 'b> TempDirBuilder<'a, 'b> {
         }
         self.builder.tempdir_in(dir)
     }
-
-    pub fn tempdir(&self) -> io::Result<TempDir> {
-        self.tempdir_in(env::temp_dir())
-    }
 }