ci: fix cargo doc

Fix the "The targets should have unique names." bug when building the doc for
the whole workspace. The problem is that the header crate might use an outdated
version of multiboot2. There is no need to build the deps here anyway, so it's
an easy fix.

.github/workflows/_build-rust.yml

@@ -94,7 +94,7 @@ jobs:
       - name: Code Style and Doc Style
         if: inputs.do-style-check
         run: |
-          cargo doc --document-private-items --features ${{ inputs.features }} --no-default-features
+          cargo doc --no-deps --document-private-items --features ${{ inputs.features }} --no-default-features
           cargo clippy --all-targets --features ${{ inputs.features }} --no-default-features
       - name: Unit Test (UNIX)
         if: inputs.do-test && runner.os != 'Windows'