Fix token used to upload releases

[traversaro]

The GitHub Action to upload release assets added in #7 to automatically upload archive in releases was working correctly on small test releases, but it was failing with the error:

##[error]Bad credentials

on the actual release workflow, that was taking more then 4 hours to complete (see for example: https://github.com/iit-danieli-joint-lab/idjl-software-dependencies-vcpkg/runs/529920938?check_suite_focus=true). It turns out that the problem here is that the automatically generated GITHUB_TOKEN secret expires after 1 hour:

The installation access token expires after 60 minutes. GitHub fetches a token for each job, before the job begins.

Note: When a workflow run or its jobs are queued for more than one hour, the token may expire before the job starts.

See https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token#about-the-github_token-secret .

For this reason, despite the examples provided in https://github.com/actions/upload-release-asset use GITHUB_TOKEN, for long jobs it make sense to use a proper generated token (see https://help.github.com/en/enterprise/2.17/user/github/authenticating-to-github/creating-a-personal-access-token-for-the-command-line). In this case, I already added a new personal token with the proper permissions in the secrets of this repo under the name ACCESS_TOKEN, and this PR makes use of it.