Update install of browser-ui-test package in CI

Author: GuillaumeGomez

src/ci/docker/host-x86_64/x86_64-gnu-tools/Dockerfile

@@ -66,14 +66,16 @@ COPY host-x86_64/x86_64-gnu-tools/checktools.sh /tmp/
 RUN curl -sL https://nodejs.org/dist/v14.4.0/node-v14.4.0-linux-x64.tar.xz | tar -xJ
 ENV PATH="/node-v14.4.0-linux-x64/bin:${PATH}"
 
+# For now, we need to use `--unsafe-perm=true` to go around an issue when npm tries
+# to create a new folder. For reference:
+# https://github.com/puppeteer/puppeteer/issues/375
+#
+# We also specify the version in case we need to update it to go around cache limitations.
+RUN npm install -g [email protected] --unsafe-perm=true
+
 ENV RUST_CONFIGURE_ARGS \
   --build=x86_64-unknown-linux-gnu \
   --save-toolstates=/tmp/toolstate/toolstates.json
 
-# Install required dependencies from browser-UI-test framework
-# For now, we need to use `--unsafe-perm=true` to go around an issue when npm tries
-# to create a new folder. For reference:
-# https://github.com/puppeteer/puppeteer/issues/375
 ENV SCRIPT /tmp/checktools.sh ../x.py && \
-  npm install -g browser-ui-test --unsafe-perm=true && \
   NODE_PATH=`npm root -g` python3 ../x.py test src/test/rustdoc-gui --stage 2