fix(build): add subtree node_modules during postinstall

Author: MarcWadai

scripts/tchap/postinstall.sh

@@ -7,6 +7,10 @@ set -x
 
 cd linked-dependencies
 pushd matrix-react-sdk
+# Normally when dealing with a subtree and node_modules, it is good practice to merge them in the parent package.json. 
+# However in our case nothing should overlaps since element and matrix-react-sdk are deeply interconnected.
+# Moreover, webpack will apply tree shaking to remove unecessary pacakges
+yarn install
 yarn unlink # :TCHAP: for local build, undo previous links if present.
 yarn link
 # yarn install --pure-lockfile