Do docs build in Nightly job

We have a separate CI job for things that require a nightly toolchain.
Building the docs requires a nightly toolchain (because of `--cfg
docsrc` flag). It makes more sense to run the docs build in the
`Nightly` job instead of hidden in the `Tests` job.

Do the docs build in the `Nightly` job instead of in the `Tests` job.

.github/workflows/rust.yml

@@ -4,7 +4,7 @@ name: Continuous integration
 
 jobs:
   Nightly:
-    name: Nightly - ASan + Bench
+    name: Nightly - ASan + Bench + Docs
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Crate
@@ -26,6 +26,10 @@ jobs:
         env:
           DO_BENCH: true
         run: ./contrib/test.sh
+      - name: Building docs
+        env:
+          DO_DOCS: true
+        run: ./contrib/test.sh
 
   Tests:
     name: Tests
@@ -43,7 +47,6 @@ jobs:
           - rust: nightly
             env:
               DO_FEATURE_MATRIX: true
-              DO_DOCS: true
           - rust: 1.29.0
             env:
               DO_FEATURE_MATRIX: true