Refactor emscripten/emsdk version to a single file

.github/workflows/ci.yml

@@ -13,7 +13,6 @@ on:
 env:
   CARGO_TERM_COLOR: always
   CARGO_INCREMENTAL: 0
-  EMSCRIPTEN_VERSION: 2.0.11
 
 jobs:
   tests:
@@ -35,6 +34,10 @@ jobs:
         run: |
           echo PATH="/usr/local/opt/gnu-tar/libexec/gnubin:$PATH" >> $GITHUB_ENV
 
+      - name: Read Emscripten version
+        run: |
+          printf 'EMSCRIPTEN_VERSION=%s\n' "$(cat emscripten-version)" >> $GITHUB_ENV
+
       - name: Cache artifacts
         id: cache
         uses: actions/cache@v2