CI: refresh toolchain matrix and scope cache keys per toolchain

Author: kipcole9

.github/workflows/ci.yml

@@ -73,9 +73,9 @@ jobs:
         cache-name: cache-elixir-deps
       with:
         path: deps
-        key: ${{ runner.os }}-mix-${{ env.cache-name }}-${{ hashFiles('**/mix.lock') }}
+        key: ${{ runner.os }}-otp${{ matrix.otp }}-elixir${{ matrix.elixir }}-mix-${{ env.cache-name }}-${{ hashFiles('**/mix.lock') }}
         restore-keys: |
-          ${{ runner.os }}-mix-${{ env.cache-name }}-
+          ${{ runner.os }}-otp${{ matrix.otp }}-elixir${{ matrix.elixir }}-mix-${{ env.cache-name }}-
 
     # Step: Define how to cache the `_build` directory. After the first run,
     # this speeds up tests runs a lot. This includes not re-compiling our
@@ -87,10 +87,9 @@ jobs:
         cache-name: cache-compiled-build
       with:
         path: _build
-        key: ${{ runner.os }}-mix-${{ env.cache-name }}-${{ hashFiles('**/mix.lock') }}
+        key: ${{ runner.os }}-otp${{ matrix.otp }}-elixir${{ matrix.elixir }}-mix-${{ env.cache-name }}-${{ hashFiles('**/mix.lock') }}
         restore-keys: |
-          ${{ runner.os }}-mix-${{ env.cache-name }}-
-          ${{ runner.os }}-mix-
+          ${{ runner.os }}-otp${{ matrix.otp }}-elixir${{ matrix.elixir }}-mix-${{ env.cache-name }}-
 
     # Step: Update dependencies
     - name: Update Ubuntu dependencies