Buildkite: install libyaml-perl when missing on PR agents

[reakaleek]

Why

• legacy_branches.pl needs Perl YAML.pm on the host agent, but some docs-ubuntu images do not have libyaml-perl
• When the module is missing, the skip guard fails open and the pipeline clones and builds anyway (for example full Kibana PR builds)

What

• Install libyaml-perl at the start of the legacy-branch check when YAML.pm is not already available

Notes

• This is a pipeline-side guard while the agent image is updated
• main/master still exit early before this install path

Made with Cursor

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