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

[Robin Westberg]

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.

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Commit messages:
 - Select Ubuntu 20.04 and GCC 10

Changes: https://git.openjdk.java.net/jdk/pull/1225/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1225&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256393
  Stats: 16 lines in 1 file changed: 3 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1225.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1225/head:pull/1225

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