RFR: 8256393: Github Actions build on Linux should define OS and GCC versions

[Robin Westberg]

On Mon, 16 Nov 2020 13:13:52 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> We should be more explicit about OS and compiler versions used in the GitHub Actions builds, to avoid problems caused by unexpected changes to the defaults. This patch changes the OS and GCC versions used from ubuntu-latest (currently 18.04, but will change to 20.04 sometime soon) / default (currently 9.3.0) to 20.04 / 10.2.0.
>
> Hold on a sec. `ubuntu-latest` is `ubuntu-18.04`, as per [GH manual](https://docs.github.com/en/free-pro-team@latest/actions/reference/specifications-for-github-hosted-runners). So this effectively upgrades the whole thing to Ubuntu 20.04, and upgrades GCC then? I think we better stick to current defaults, i.e. `ubuntu-18.04` and its GCC. 
> 
> In JDK-8256277, we did not upgrade MacOS target either...

Right, currently ubuntu-latest means 18.04, but that is only true for another two weeks (see https://github.com/actions/virtual-environments/issues/1816 - originally it was also planned for next week). So I think we should go straight for the upcoming latest.

> .github/workflows/submit.yml line 91:
> 
>> 89: 
>> 90:       - name: Build jtreg
>> 91:         run: sh make/build-all.sh ${JAVA_HOME_8_X64}
> 
> What is this change?

On ubuntu-20.04 the default Java installation is now set to 11, but jtreg still requires Java 8 for building.

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PR: https://git.openjdk.java.net/jdk/pull/1225