Rework GitHub Actions workflow configuration (#4412)
Rework GitHub Actions workflow configuration (#4412)

- Make jobs run on latest versions of the OS's (except for
  "Checks", which relies on specific versions of checkers coming
  with Ubuntu 18.04)
- Bump actions/checkout to v2
- Rewrite `apt-get` to `apt` (almost the same, but `apt` is the
  2nd gen tool)
- Drop step names (they took too much vertical space, didn't add
  too much useful information, and were very typo-prone)
- Move superfluous environment variables (e.g., `BUILD_OPTIONS` or
  `SKIP_LIST`) to their expansion sites (use YAML block scalars
  `>-` where appropriate to enhance readability)
- Fix duplicate installation of pylint in the "Checks" job

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu
akosthekiss/jerryscript,bzsolt/jerryscript,jerryscript-project/jerryscript,jerryscript-project/jerryscript,robertsipka/jerryscript,jerryscript-project/jerryscript,zherczeg/jerryscript,robertsipka/jerryscript,bzsolt/jerryscript,zherczeg/jerryscript,robertsipka/jerryscript,akosthekiss/jerryscript,dbatyai/jerryscript,akosthekiss/jerryscript,jerryscript-project/jerryscript,robertsipka/jerryscript,bzsolt/jerryscript,robertsipka/jerryscript,akosthekiss/jerryscript,akosthekiss/jerryscript,dbatyai/jerryscript,dbatyai/jerryscript,zherczeg/jerryscript,dbatyai/jerryscript,robertsipka/jerryscript,zherczeg/jerryscript,jerryscript-project/jerryscript,bzsolt/jerryscript,zherczeg/jerryscript,bzsolt/jerryscript,dbatyai/jerryscript