nixguix: Add support for pseudo url with missing schema

Related to swh-loader-core#3294 (closed) Related to T3781 Depends on !432 (closed)

---

Migrated from D8612 (view on Phabricator)

swh/lister/nixguix/tests/data/nixpkgs-swh_sources.json

@@ -29,6 +29,13 @@
       ],
       "integrity": "sha256-bss09x9yOnuW+Q5BHHjf8nNcCNxCKMdl9/2/jKSFcrQ="
     },
+    {
+      "type": "url",
+      "urls": [
+        "www.roudoudou.com/export/cpc/rasm/rasm_v0117_src.zip"
+      ],
+      "integrity": "sha256-wAEswtkl3ulAw3zq4perrGS6Wlww5XXnQYsEAoYT9fI="
+    },
     {
       "type": "url",
       "urls": [