ci: fix wasm32 broken due to a NodeJS version bump

Emscripten's SDK recently bumped the version of NodeJS they shipped, but
our Dockerfile for the wasm32 builder hardcoded the version number. This
will cause consistent CI failures once the currently cached image is
rebuilt (either due to a change or due to the cache expiring).

This commit fixes the problem by finding the latest version of NodeJS in
the Emscripten SDK and symlinking it to a "latest" directory, which is
then added to the PATH.

src/ci/docker/wasm32/Dockerfile

@@ -25,7 +25,18 @@ RUN ln `which python3` /usr/bin/python
 
 ENV PATH=$PATH:/emsdk-portable
 ENV PATH=$PATH:/emsdk-portable/upstream/emscripten/
-ENV PATH=$PATH:/emsdk-portable/node/12.9.1_64bit/bin/
+
+# Rust's build system requires NodeJS to be in the path, but the directory in
+# which emsdk stores it contains the version number. This caused breakages in
+# the past when emsdk bumped the node version causing the path to point to a
+# missing directory.
+#
+# To avoid the problem this symlinks the latest NodeJs version available to
+# "latest", and adds that to the path.
+RUN ln -s /emsdk-portable/node/$(ls /emsdk-portable/node | sort -V | tail -n 1) \
+          /emsdk-portable/node/latest
+ENV PATH=$PATH:/emsdk-portable/node/latest/bin/
+
 ENV BINARYEN_ROOT=/emsdk-portable/upstream/
 ENV EMSDK=/emsdk-portable
 ENV EM_CONFIG=/emsdk-portable/.emscripten