Error in OctoInstaller task: unable to verify the first certificate

[danielsomervillepc]

When running the OctoInstaller task it now errors as below:

==============================================================================
Task         : Octopus CLI Installer
Description  : Install a specific version of the Octopus CLI (C#)
Version      : 5.0.1071
Author       : Octopus Deploy
Help         : Version: 6.0.1071. [More Information](https://g.octopushq.com/TFS-VSTS)
==============================================================================
##[error]"Failed to install octo v7.4.3190.
unable to verify the first certificate
Error: unable to verify the first certificate
    at TLSSocket.onConnectSecure (_tls_wrap.js:1088:34)
    at TLSSocket.emit (events.js:198:13)
    at TLSSocket._finishInit (_tls_wrap.js:666:8)
Finishing: OctoInstaller

It appears that the site certificate for g.octopushq.com was updated this week, and is missing the intermediate certificate. Google Chrome on Windows doesn't report any problem, but Node.js and the openssl s_client command on Ubuntu both refuse to connect to the server.