Convince GitLab not to crop collapsed multiline strings (#3971)
Use of `- >` instead of `- |` workarounds GitLab quirk when it crops collapsed multiline `script:` section commands in its CI job logs. This PR also fixes `- |` based `script:` steps to behave properly after `- >` conversion. Resolves https://github.com/paritytech/ci_cd/issues/972.