rustdoc-search: fix a race condition in search index loading

`var` declare it in the global scope, and `const` does not.
It needs to be declared in global scope.

Author: notriddle

src/librustdoc/html/render/write_shared.rs

@@ -297,7 +297,7 @@ pub(super) fn write_shared(
                     .replace("\\\"", "\\\\\"")
             ));
             all_sources.sort();
-            let mut v = String::from("const srcIndex = new Map(JSON.parse('[\\\n");
+            let mut v = String::from("var srcIndex = new Map(JSON.parse('[\\\n");
             v.push_str(&all_sources.join(",\\\n"));
             v.push_str("\\\n]'));\ncreateSrcSidebar();\n");
             Ok(v.into_bytes())
@@ -317,7 +317,7 @@ pub(super) fn write_shared(
     // with rustdoc running in parallel.
     all_indexes.sort();
     write_invocation_specific("search-index.js", &|| {
-        let mut v = String::from("const searchIndex = new Map(JSON.parse('[\\\n");
+        let mut v = String::from("var searchIndex = new Map(JSON.parse('[\\\n");
         v.push_str(&all_indexes.join(",\\\n"));
         v.push_str(
             r#"\