Fix env var

Author: dhardy

.github/workflows/test.yml

@@ -20,7 +20,7 @@ jobs:
           override: true
         run: cargo install cargo-deadlinks
       - name: doc (rand)
-        env: RUSTDOCFLAGS: --cfg doc_cfg
+        env: RUSTDOCFLAGS: '--cfg doc_cfg'
         # --all builds all crates, but with default features for other crates (okay in this case)
         run: cargo deadlinks --ignore-fragments -- --all --features nightly,serde1,getrandom,small_rng