ci fails to install neume-network/strategies as PGP keys aren't configured

[TimDaub]

• See this log: https://github.com/neume-network/core/runs/6914086794?check_suite_focus=true#step:4:10

  Cloning into '/home/runner/work/core/core/src/strategies'...
  git@github.com: Permission denied (publickey).
  fatal: Could not read from remote repository.

Cloning the strategies repository fails as we haven't setup PGP keys to use GitHub with SSH in Actions.

• However, using https to pull the repo as default and is suggest here: https://github.com/neume-network/core/commit/b9746126d4e6758f3b5b19175015d5888a3f6437 should also not be an option. We want to use PGP, but for CI to pass maybe we could replace SSH with HTTPS

[TimDaub]

• over on neume-network/data I successfully got installation to run by setting up an SSH key with the GitHub Action: https://github.com/neume-network/data/runs/7087831171?check_suite_focus=true
• https://github.com/webfactory/ssh-agent#usage
• https://github.com/neume-network/data/blob/e56fb747238652603435d3e807b513454f719f0e/.github/workflows/node.js.yml