CI: Fix: Requests to crates.io need extra header

Fixes #4.

Author: alexdewar

mlc_config.json

@@ -0,0 +1,10 @@
+{
+    "httpHeaders": [
+        {
+            "urls": ["https://crates.io/"],
+            "headers": {
+                "Accept": "text/html"
+            }
+        }
+    ]
+}