Add output parameter with path to installed JDK

[tbroyer]

When calling the action several times, it will overwrite the JAVA_HOME environment variable and prepend it to the PATH, effectively shadowing a previous run of the action.
Fortunately, it will also set a JAVA_HOME_<version>_<architecture> environment variable, but this could be more idiomatic as an output parameter (in addition to the environment variable, which can also be useful on its own: #44 (comment))

This would be a great addition to #21

That way, one could write something like:

- name: Setup JDK 11
  id: setup-java-11
  uses: actions/setup-java@v1
  with:
    java-version: 11
    # here with #21, one would disable "installation"
- name: Setup JDK 14
  id: setup-java-14
  uses: actions/setup-java@v1
  with:
    java-version: 14
    # here with #21, one would disable "installation"
- name: Setup JDK 8
  # Ran last due each run shadowing the previous ones; see also #21
  uses: actions/setup-java@v1
  with:
    java-version: 8
- name: Build and run tests (with JDK 8)
  run: ./gradlew build
- name: Run tests with JDK 11
  run: ./gradlew test -Ptest.jdk-path="${{ steps.setup-java-11.outputs.jdk-path }}"
- name: Run tests with JDK 14
  run: ./gradlew test -Ptest.jdk-path="${{ steps.setup-java-14.outputs.jdk-path }}"

(here, the build is still running with JDK 8, but it coud configure its tests to actually use another JDK)

[tbroyer]

This is actually worse than that: java-version: 11 won't actually set JAVA_HOME_11_x64, but JAVA_HOME_11.0.7_x64, making that variable essentially useless as its name will actually change whenever a new version of the JDK is released.

As a workaround, I'm using (with a previous call with java-version: 11 and a previous Gradle build with JDK 11: the new build here still runs with JDK 11 so it won't recompile Java sources)

      # Workaround for https://github.com/actions/setup-java/issues/65
      - name: Backup JAVA_HOME
        id: java-home
        run: echo "::set-output name=path::$JAVA_HOME"

I could have used a ::set-env too of course to set JAVA_HOME_11_x64.

[Ryan9Manu]

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[Ryan9Manu]

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[Ryan9Manu]

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