[Nix] Explicitly set emscripten cache

Summary:
Avoid emscripten trying to write to the nix store, which
will always fail

Depends on D8395

Test Plan:
```
cd web

yarn build-db-wasm
```

Reviewers: atul, jon

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8446

scripts/source_development_defaults.sh

@@ -43,8 +43,10 @@ export TMP=/tmp/app.comm
 export TEMP="$TMP"
 export TMPDIR="$TMP"
 export TEMPDIR="$TMP"
+export EM_CACHE="$TMP/em_cache"
+
+mkdir -p "$EM_CACHE"
 
-mkdir -p "$TMP"
 
 # For cargo + rustup applications, ensure cargo user bin directory is on path
 if [[ ! "$PATH" =~ \.cargo/bin ]]; then