Fix RustDoc generation. (#3763)

* Attemp to fix it.

* Move env to the begining.

substrate/.gitlab-ci.yml

@@ -288,7 +288,7 @@ build-rust-doc-release:
   <<:                              *build-only
   script:
     - rm -f ./crate-docs/index.html # use it as an indicator if the job succeeds
-    - BUILD_DUMMY_WASM_BINARY=1 time RUSTDOCFLAGS="--html-in-header $(pwd)/rustdoc-header.html" cargo +nightly doc --release --all --verbose
+    - BUILD_DUMMY_WASM_BINARY=1 RUSTDOCFLAGS="--html-in-header $(pwd)/rustdoc-header.html" time cargo +nightly doc --release --all --verbose
     - cp -R ./target/doc ./crate-docs
     - echo "<meta http-equiv=refresh content=0;url=substrate_service/index.html>" > ./crate-docs/index.html
     - sccache -s