Predownload native compilers in CI

[blitz-1306]

Preface

This PR tweaks CI to use compiler downloading script prior to executing tests. It will help us to avoid ETXTBSY exit codes, probably caused by attempts to access compilers that are still downloading. We can reconfigure to use docker image later, when we decide where to publish it.

Changes

• Added CI commands to predownload native compilers and set ENV to use them instead of downloading on-demand.

Notes

Part of the code, that is responsible for downloading on-demand will not be covered by the tests as the result of predownloading. Other option is to create a test that downloads only one compiler that is not used by other tests (or remove one rarely used compiler).

Regards.

[codecov-commenter]

Codecov Report

Merging #95 (2fb850f) into master (b736fb5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #95   +/-   ##
=======================================
  Coverage   92.20%   92.20%           
=======================================
  Files         254      254           
  Lines        5197     5197           
  Branches      819      819           
=======================================
  Hits         4792     4792           
  Misses        254      254           
  Partials      151      151           

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[cd1m0]

lgtm!