Look for gradle wrapper in build-root-directory by default

This removes the need to specify `wrapper-directory` when using a Gradle
project that is not located in the root of the workspace.

Fixes #44.

.github/workflows/prod.yml

@@ -17,11 +17,18 @@ jobs:
     - name: Test wrapper
       uses: ./
       with:
-        wrapper-directory: __tests__/data/basic
         build-root-directory: __tests__/data/basic
         dependencies-cache-enabled: true
         configuration-cache-enabled: true
         arguments: test
+    - name: Test custom wrapper location
+      uses: ./
+      with:
+        build-root-directory: __tests__/data/basic
+        wrapper-directory: __tests__/data/basic
+        dependencies-cache-enabled: false
+        configuration-cache-enabled: false
+        arguments: test
     - name: Test dist download
       uses: ./
       with: