Skip duplicated Github actions run

[dolfinus]

Hello.

There is an annoying fact about Github Actions - sometimes it runs actions twice. For example:

1. You just pushed a commit to a branch - testing workflow will run only once
2. Someone forked your repo and creates a pull request - same thing
3. You've created a new branch (feature, bugfix or release branch), pushed a commit here and also created a pull request to the main branch - two copies of workflow will be run

Running a workflow twice does not make any sense, it just takes twice the time without any useful side effect.

In this PR I've added a separated job of checking if another copy of workflow already started: https://github.com/fkirc/skip-duplicate-actions

This job allows to:

1. Skip the second copy of workflow in the case 3 (branch in original repo + PR to main branch). Cases 1 and 2 will work just the same as now
2. Skip workflow if someone changed only files which are not connected to package code, like README or CHANGELOG
3. Cancel already started workflow in the same PR or branch if someone pushed new commits to it.

Also I've found another quite annoying bug - step actions/cache does not update cache item if it was restored using a primary key. For example, some dependencies like dialyzer or other ones store their files in deps or _build folders (which are cached in the workflow). But when these dependencies perform some updates of these folders, it does not lead to the updating of mix.lock file. File hasn't been changed -> hash is the same -> cache primary key is the same -> caching action will not update the existing cache. This will cause the increase of workflow run time because the stored cache will not be used by some steps. Until mix.lock will be updated which will lead to updating the cache item. Here I've appended a commit SHA to the cache primary key, so it will always be updated. But it will not affect cache reading.

What do you think about that?