ci: version the deps cache key

Beams compiled by one Elixir/OTP pair can't be cover-instrumented by
another; coverage was picking up the 1.19/OTP 26 build. Include both
versions in the cache key.

Author: gilbertwong96

.github/workflows/ci.yml

@@ -32,7 +32,7 @@ jobs:
           path: |
             deps
             _build
-          key: ${{ runner.os }}-mix-${{ hashFiles('**/mix.lock') }}
+          key: ${{ runner.os }}-mix-1.20-27-${{ hashFiles('**/mix.lock') }}
           restore-keys: ${{ runner.os }}-mix-
 
       - uses: actions/cache@v4
@@ -64,7 +64,7 @@ jobs:
           path: |
             deps
             _build
-          key: ${{ runner.os }}-mix-${{ hashFiles('**/mix.lock') }}
+          key: ${{ runner.os }}-mix-1.20-27-${{ hashFiles('**/mix.lock') }}
           restore-keys: ${{ runner.os }}-mix-
 
       - name: Fetch dependencies