Fix installer to use correct branch dir

[UniMa007]

Description

Running the regular installer from the documentation on https://docs.origintrail.io/dkg-v8-current-version/v8-dkg-core-node/run-a-v8-core-node-on-testnet/v8-dkg-core-node-installation#id-2.-download-origintrail-v8-dkg-core-node-installer will fail.

This fixes #3674

It seems like the installer does a few things wrong:

Fixes # (issue)

• The installer will download now the v6 version , but look into the wrong branch directory.
  -> Fixed to use the correct dir

• The installer will somehow forget to add the required repositories public-current and private current into the .origintrail_noderc

• This is fixed in the installer.sh to create also those two entries

• The config.json for TESTNET is configured to check the v8/release/testnet branch to check whether it should update. Since the version on this branch is 8.1.2 it will update to the broken node version, that will cause again the same issue when running the initial script

-> I changed the config to use v6/release/testnet. However this only works, as soon as this change is merged to v6/release/testnet

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A
• Test B

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules