CI: Also set `RUSTDOCFLAGS`

`rustdoc` doesn't support all the same flags as `rustc`, so it doesn't
listen to `RUSTFLAGS` and has its own env variable. We are only setting
flags that both understand.

.github/workflows/ci.yml

@@ -27,6 +27,7 @@ jobs:
             extra_flags: zfuture
     env:
       RUSTFLAGS: ${{ matrix.rustflags }}
+      RUSTDOCFLAGS: ${{ matrix.rustflags }}
 
     steps:
       - uses: actions/checkout@v4