Make url overrides for downloading toolchains work.

Previously, the default url template for downloading toolchains
was always present as the first place to download tools from. This
prevents overriding the default location for the tools so that an
internal mirror can be used.

Fixes bazelbuild#867

rust/private/repository_utils.bzl

@@ -526,8 +526,7 @@ def load_arbitrary_tool(ctx, tool_name, tool_subdirectories, version, iso_date,
     # View the indices mentioned in the docstring to find the tool_suburl for a given
     # tool.
     tool_suburl = produce_tool_suburl(tool_name, target_triple, version, iso_date)
-    static_rust = ctx.os.environ.get("STATIC_RUST_URL", "https://static.rust-lang.org")
-    urls = ["{}/dist/{}.tar.gz".format(static_rust, tool_suburl)]
+    urls = []
 
     for url in getattr(ctx.attr, "urls", DEFAULT_STATIC_RUST_URL_TEMPLATES):
         new_url = url.format(tool_suburl)