chore: remove RUSTFLAG from docs workflow

`"--cfg docsrs"` shouldn't be part of `RUSTFLAGS` but only of `RUSTDOCFLAGS`.

This should fix failing docs Actions.

Pull-Request: #5517.

Author: guillaumemichel

.github/workflows/docs.yml

@@ -17,7 +17,6 @@ jobs:
       run: cargo +nightly doc --no-deps --workspace -F full
       env:
         RUSTDOCFLAGS: "--cfg docsrs"
-        RUSTFLAGS: "--cfg docsrs"
     - name: Add index file
       run: |
         mkdir host-docs