ci: Deny cargo doc warnings

A job should fail whenever `rust doc` emits warnings.

.radicle/ambient.yaml

@@ -16,6 +16,7 @@ plan:
       export CARGO_HOME=/workspace/deps
       export HOME=/root
       export PATH="/root/.cargo/bin:$PATH"
+      export RUSTDOCFLAGS='-D warnings'
 
       cargo doc --workspace --no-deps
 

.radicle/native.yaml

@@ -1,4 +1,6 @@
 shell: |
+  export RUSTDOCFLAGS='-D warnings'
+
   cargo --version
   rustc --version