Re-name nightly CI job to Nightly

In line with the `Tests` job and for the fact that this job does stuff
with the nightly toolchain other than bench.

Re-name nightly CI job from `bench_nightly`to `Nightly`.

Author: tcharding

.github/workflows/rust.yml

@@ -3,7 +3,7 @@ on: [push, pull_request]
 name: Continuous integration
 
 jobs:
-  bench_nightly:
+  Nightly:
     name: Nightly - ASan + Bench
     runs-on: ubuntu-latest
     strategy: