Use javascript-action instead of platform dependent Shell commands

[juergenhoetzel]

This change allows to execute github installation test in a platform-agnostic way (required to merge #35 by @ikappak)

Instead of a simple "test -x eldev" test, eldev is actually
bootstrapped again to verify that the net-installation was indeed
successful.

The previous shell tests also use the hardcoded path
https://raw.github.com/doublep/eldev/master/bin/eldev to download the
eldev scripts. So CI will not test changes introduced in the PRs.

The utility function getRawUrl generates the correct raw download
URL depending on the context (pushing to master or PR) and also adds a
platform dependent suffix (.bat on Windows) to the script name to make
the script download/testing platform-agnostic.

[juergenhoetzel]

I have testet the functionality in dummy-repo https://github.com/juergenhoetzel/github-actions-test/pull/3

[doublep]

Looks fine (I will trust you that it works like described, my JS and GitHub knowledge is limited). Please squash the two commits, they essentially do one thing together. And also a minor nitpick: please reformat if (...) do_something on two lines. I occasionally use this myself in Lisp when it is pure evaluation, but dislike single-line formatting in all other cases, and especially when condition is not simple.

[juergenhoetzel]

Looks fine (I will trust you that it works like described, my JS and GitHub knowledge is limited). Please squash the two commits, they essentially do one thing together. And also a minor nitpick: please reformat if (...) do_something on two lines. I occasionally use this myself in Lisp when it is pure evaluation, but dislike single-line formatting in all other cases, and especially when condition is not simple.

Thanks for your quick and helpful response. I just squashed and rebased the commits.

[doublep]

Merged, thank you.