[1.9.x] Fix for forked PRs.

[scottarnette]

Change Description

We have noticed several PRs from forked repositories are failing in Travis. This was determined to be caused by Travis builds being unable to determine the dependent eosio and eosio.cdt versions via the Github API.

Fixed CI issues regarding forked PRs:

• If the build source is not Buildkite, we fall back to using other methods to determine dependencies as defined in pipeline.jsonc.
  • For the EOSIO dependency, this is easy because we already push docker images named by both branch name and commit hash. Buildkite still determines the latest commit hash if the dependency is a branch while Travis will just use the latest branch image if defined that way.
  • For the CDT dependency, this is a bit brute-forced since CDT binaries are stored by a shorted commit hash. This now clones the eosio.cdt repository and checkouts the defined dependency (branch or commit hash). Then, git rev-parse --verify HEAD is executed to determine the HEAD commit hash, just incase the dependency is referenced by branch. I am open to other suggestions for determining the CDT binary to install.

See:
https://travis-ci.com/EOSIO/eosio.contracts/builds/135300601 | Travis build non-forked.
https://travis-ci.com/EOSIO/eosio.contracts/builds/135301926 | Travis build from forked PR.
https://buildkite.com/EOSIO/eosio-dot-contracts/builds/784 | Unaffected Buildkite build.

Deployment Changes

• Deployment Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions