Make rust doc deterministic

Avoid committing if there is no change compared to the previous version,
to keep history clean.

Ref project-oak#246

.github/workflows/build_rust_docs.yaml

@@ -58,5 +58,6 @@ jobs:
         run: |
           cd ./out
           git add .
+          [[ -z "$(git status --porcelain)" ]] && (echo 'no changes to commit'; exit 0)
           git commit --message="Update gh-pages from ${GITHUB_SHA}"
           git push

scripts/build_gh_pages

@@ -35,6 +35,9 @@ cargo doc --no-deps --target-dir="${TARGET_DIR}"
 # Remove non-doc artifacts from the target dir.
 rm --recursive --force "${TARGET_DIR}/debug"
 
+# Remove non-deterministic files.
+rm "{TARGET_DIR}/.rustc_info.json"
+
 # The docs generated from the Cargo workspace do not include a workspace-level index, so we generate
 # one here and redirect to the Oak SDK documentation.
 cat <<-END > "${TARGET_DIR}/index.html"