chore(ci): ignore libc from renovate updates

We have downgraded this in rust-lang#15851
We should also let renovate ignore it.

Author: weihanglo

.github/renovate.json5

@@ -8,6 +8,7 @@
   ignorePaths: [
     '**/tests/**',
   ],
+  ignoreDeps: ['libc'],
   customManagers: [
     {
       customType: 'regex',