Code escrow.

You factor in the expense of having your code releases escrowed by a third party (where part of the escrow contract itself is: "must be buildable from sources as provided"), and have a post-release pipeline that automatically uploads the new version. At the end of the term, the escrow holder releases all the versions.

This is a fairly common arrangement in high finance. If you want to supply services to a bank/insurer/etc. they will typically require an escrow arrangement as a contingency plan against you as a vendor going away. And yes, they pay the escrow costs.