fix: further downgrade mermaid, internal dep env problem (mozilla-services#1974)

fix: further downgrade mermaid, internal dep env problem

Author: taddes

.github/workflows/publish-docs.yaml

@@ -14,7 +14,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     env:
-      MDBOOK_ENV: 0.5.0 # Current mdbook version. See `https://crates.io/crates/mdbook`.
+      MDBOOK_ENV: 0.4.51 # Current mdbook version. See `https://crates.io/crates/mdbook`.
       MERMAID_ENV: 0.15.0 # For crate `mdbook-mermaid`. See: `https://github.com/badboy/mdBook-mermaid/`.
       DEST_DIR: /home/runner/.cargo/bin
     steps: