RFR: 8256277: Github Action build on macOS should define OS and Xcode versions

[Aleksey Shipilev]

On Fri, 13 Nov 2020 11:48:31 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 Xcode versions used from latest (currently 10.15) / default (currently 12.0) to 10.15 / 11.3.1.

Looks fine to me. I was wondering if we should do the same for `ubuntu-latest` and `windows-latest`.

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1201