silence warning if NIX_INDEX_DATABASE is set

Related to nix-community/nix-index-database#92

src/index.rs

@@ -21,6 +21,12 @@ pub fn check_database_exists() {
 
 /// Prints a warning if the nix-index database is out of date.
 pub fn check_database_updated() {
+    if os::getenv("NIX_INDEX_DATABASE").is_some() {
+        // If the user has set NIX_INDEX_DATABASE, they are responsible for keeping it up to date
+        // because if it's part of the nix store, the timestamp be always 1970-01-01.
+        // This environment variable is set by nix-index-database.
+        return;
+    }
     let database_file = get_database_file();
     if is_database_old(&database_file) {
         eprintln!(