Add additional information to nightly job

As we do in `rust-secp256k1` use a string that includes 'Nightly' in
name of the nightly test.

Author: tcharding

.github/workflows/rust.yml

@@ -27,7 +27,7 @@ jobs:
         run: ./contrib/test.sh
 
   Nightly:
-    name: Bench + Docs + Fmt
+    name: Nightly - Bench + Docs + Fmt
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Crate