RFR: 8256393: Github Actions build on Linux should define OS and GCC versions [v2]

[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.

Robin Westberg has updated the pull request incrementally with one additional commit since the last revision:

  Select gcc/g++ package versions explicitly

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1225/files
  - new: https://git.openjdk.java.net/jdk/pull/1225/files/87ab0729..3dcb26d6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1225&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1225&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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