Run Windows test with JDK 8 rather than JDK 11

The initial goal (IIRC) was to make sure that the build
could run with JDK 11, but unfortunately this means that
on Windows the JDK 8-specific -Xbootclasspath/p is not
execised, whereas this is probably the most important
thing to test for Windows compatibility, given that it
contains file paths.

With this change, the build itself (not the code) is no
longer exercised against JDK 11, but it doesn't really
matter actually.

.github/workflows/gradle.yaml

@@ -11,7 +11,7 @@ on:
 env:
   GRADLE_OPTS: "-Dorg.gradle.daemon=false"
 
-# First build on Linux with JDK 8 and Windows with JDK 11
+# First build on Linux and Windows with JDK 8
 # Then run tests with various Gradle and JDKs versions (Linux only)
 jobs:
   linux:
@@ -45,7 +45,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Setup environment
-        run: echo "JAVA_HOME=${Env:JAVA_HOME_11_X64}" | Tee-Object -FilePath $env:GITHUB_ENV -Append
+        run: echo "JAVA_HOME=${Env:JAVA_HOME_8_X64}" | Tee-Object -FilePath $env:GITHUB_ENV -Append
 
       - name: Build with Gradle
         id: gradle-build