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

[Aleksey Shipilev]

On Mon, 16 Nov 2020 12:51:25 GMT, Robin Westberg <rwestberg 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...

.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?

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Changes requested by shade (Reviewer).

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