Cache ~/.konan on CI

See: https://kotlinlang.org/docs/native-improving-compilation-time.html#general-recommendations

.github/workflows/ci.yaml

@@ -13,6 +13,13 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
+      - uses: actions/cache@v1
+        with:
+          path: ~/.konan
+          key: ${{ runner.os }}-konan-${{ hashFiles('**/*.gradle.kts') }}-${{ hashFiles('buildSrc/src/main/kotlin/Versions.kt') }}
+          restore-keys: |
+            ${{ runner.os }}-konan-
+
       - uses: actions/cache@v1
         with:
           path: ~/.gradle/wrapper