Figure out why pkg-config doesn't work as expected for Expat

[mdaines]

In https://github.com/mdaines/viz-js/commit/69aa8a8b7fd5e1b6ab4d49a72e186b4ff245c683 I had to work around the Graphviz configure script reporting that it couldn't find pkg-config when configuring for Expat. According to the Emscripten docs emconfigure is supposed to set up environment variables for pkg-config.

Graphviz commit that changed how Expat is detected: https://gitlab.com/graphviz/graphviz/-/commit/4d530ac825ddfba3d0b0c130aa0cacf2801e80fb

Ideally the Dockerfile wouldn't have to specify EXPAT_CFLAGS and EXPAT_LIBS explicitly.