Setup fails for PRs from forked repos

[prajakta-gokhale]

Noticed this issue in ros-tooling/cross_compile#37.

In this PR, the base branch is from a fork of the repo where the action runs.
PR base branch: https://github.com/aws-ros-dev/cross_compile/tree/allabana/doc-fixes
Main repo: https://github.com/ros-tooling/cross_compile

Instead of cloning the fork and checking out the source branch, the action looks for the branch in the parent repo. When it is unable to find the branch, the test errors out.

Test log output:

repositories:
  cross_compile:
    type: git
    url: "https://github.com/ros-tooling/cross_compile.git"
    version: "allabana/doc-fixes"
EOF
=== src/cross_compile (git) ===
Could not checkout ref 'allabana/doc-fixes': error: pathspec 'allabana/doc-fixes' did not match any file(s) known to git
##[error]The process 'bash' failed with exit code 1
##[error]Node run failed with exit code 1

[piraka9011]

@prajakta-gokhale could you close this and open it up in action-ros2-ci instead?

The failure is from this command

[piraka9011]

Reopened here