Update fuzz CI job to use our MSRV rather than 1.58

Author: TheBlueMatt

.github/workflows/build.yml

@@ -176,7 +176,7 @@ jobs:
   fuzz:
     runs-on: ubuntu-latest
     env:
-      TOOLCHAIN: 1.58
+      TOOLCHAIN: 1.63
     steps:
       - name: Checkout source code
         uses: actions/checkout@v3