Include otp and elixir versions in cache

Author: martosaur

.github/workflows/ci.yml

@@ -31,8 +31,8 @@ jobs:
       uses: actions/cache@v2
       with:
         path: deps
-        key: ${{ runner.os }}-mix-${{ hashFiles('**/mix.lock') }}
-        restore-keys: ${{ runner.os }}-mix-
+        key: ${{ runner.os }}-${{ matrix.elixir }}-${{ matrix.otp }}-mix-${{ hashFiles('**/mix.lock') }}
+        restore-keys: ${{ runner.os }}-${{ matrix.elixir }}-${{ matrix.otp }}-mix-
 
     - run: mix deps.get
     - run: mix test