[DEVOPS-643] ci: add buildkite-agent to PATH
[DEVOPS-643] ci: add buildkite-agent to PATH

Needs to be added to PATH because nix-shell removes any /nix/store
PATH entries.

input-output-hk/daedalus,input-output-hk/daedalus,input-output-hk/daedalus,input-output-hk/daedalus,input-output-hk/daedalus,input-output-hk/daedalus