Add nix to Dependabot's supported ecosystems (SchemaStore#5534)

https://github.blog/changelog/2026-04-07-dependabot-version-updates-now-support-the-nix-ecosystem/

Author: JamieMagee

src/schemas/json/dependabot-2.0.json

@@ -691,6 +691,7 @@
         "julia",
         "maven",
         "mix",
+        "nix",
         "npm",
         "nuget",
         "opentofu",