fix(express): use yarn to run build script
fix(express): use yarn to run build script

`tsc` is now installed in the root node_modules, so npm can't find it
anymore. Instead, use yarn to run `tsc` since it knows about the
binaries installed in the root.

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS