Doc CI: fix expansion of env-var

Author: Bromeon

.github/workflows/doc.yml

@@ -53,7 +53,7 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: doc
-          args: -p gdnative --lib --no-deps --features ${GDRUST_FEATURES}
+          args: -p gdnative --lib --no-deps --features ${{ env.GDRUST_FEATURES }}
 
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       # As search-index.js changes every time, even if source hasn't changed, this will not need 'git commit --allow-empty'