Bug: `verify_treehashes` throws an error if the current working directory is not the repository root

[DilumAluthge]

If the current working directory is the repository root, verify_treehashes works fine:

root@docker:~/workspace/julia# pwd
/root/workspace/julia
root@docker:~/workspace/julia# ../cryptic-buildkite-plugin/bin/verify_treehashes
Repository location: .
[./.buildkite/pipelines/main/0_webui.yml] -> .buildkite/pipelines/main/misc/signed_pipeline_test.yml: ✔️
[./.buildkite/pipelines/scheduled/0_webui.yml] -> .buildkite/pipelines/scheduled/coverage/coverage_linux64.yml: ✔️
root@docker:~/workspace/julia#

But if the current working directory is NOT the repository root, verify_treehashes errors:

root@docker:~/workspace# pwd
/root/workspace
root@docker:~/workspace# ls
cryptic-buildkite-plugin  julia
root@docker:~/workspace# cryptic-buildkite-plugin/bin/verify_treehashes
Repository location: julia
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/experimental/0_webui.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/experimental/launch_unsigned_builders.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/0_webui.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/launch_unsigned_builders.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/misc/doctest.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/misc/embedding.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/misc/llvmpasses.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/misc/sanitizers.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/misc/signed_pipeline_test.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/misc/whitespace.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/platforms/package_linux.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/main/platforms/tester_linux.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/scheduled/0_webui.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/scheduled/coverage/coverage_linux64.yml: No such file or directory
/root/workspace/cryptic-buildkite-plugin/lib/yaml_extraction_prologue.sh: line 83: julia/.buildkite/pipelines/scheduled/launch_unsigned_jobs.yml: No such file or directory