std: Refactor _var function to use env_imp::getenv

Author: xtqqczze

library/std/src/env.rs

@@ -224,7 +224,7 @@ pub fn var<K: AsRef<OsStr>>(key: K) -> Result<String, VarError> {
 }
 
 fn _var(key: &OsStr) -> Result<String, VarError> {
-    match var_os(key) {
+    match env_imp::getenv(key) {
         Some(s) => s.into_string().map_err(VarError::NotUnicode),
         None => Err(VarError::NotPresent),
     }