elastic-ci-stack-s3-secrets-hooks: soft fail when no SSH keys

[pda]

Brings in buildkite/elastic-ci-stack-s3-secrets-hooks#32:

Warn instead of fail when no SSH keys found in S3

Don't hard-fail when there's no SSH keys found. The hard-fail was intended to make debugging more obvious, rather than an obscure permission error during git clone. However there are legitimate use-cases for enabling this secrets plugin but not fetching an SSH key, for example any combination of:

• git config url.<base>.insteadOf can be used to rewrite a git@... repo to https://... with HTTPS credentials sourced some other way,
• a pipeline may be using the environment part of this secrets plugin, but not the SSH key part.
• an agent may run some pipelines that use this secrets plugin, and some that don't.

This patch;

• removes the exit 1 hard failure,
• adjusts the BUILDKITE_REPO pattern to not match non-SSH git:// URLs,
• elaborates on the warning message, with a link to documentation.

Closes #24

(Plus a README typo fix)