ci: fix RUSTDOCFLAGS syntax for Windows PowerShell

Inline env var assignment (VAR=val cmd) is bash syntax only.
PowerShell rejects it. Use the step-level env: block instead,
which GitHub Actions handles cross-platform.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: crumplecup

.github/workflows/ci.yml

@@ -60,7 +60,9 @@ jobs:
         run: cargo clippy --workspace --all-targets ${{ matrix.extra_exclude }} -- -D warnings
 
       - name: Doc check
-        run: RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps ${{ matrix.extra_exclude }}
+        env:
+          RUSTDOCFLAGS: "-D warnings"
+        run: cargo doc --workspace --no-deps ${{ matrix.extra_exclude }}
 
   # Ubuntu only: test elicitation_derive (fast, no heavy deps).
   test-derive: