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.
There are a few such services around, usually owned by a giant global consulting house.
The idea is that if you as a vendor go out of business or otherwise become unable to maintain the software, the finance institution gets access to the software via the escrow. Importantly, they also gain the contractual and legal rights to further maintain (read: modify) the software.
Under such contract the vendor has an obligation to upload periodic code releases to the escrow service, and the escrow service validates that the release builds. (And passes the bnudled test suite.) Rather surprisingly these services don't even cost that much... at least in the grand scheme of things. The requirement usually comes up only when the underlying supplier deal is at least six figures annually.
ß: well, contract law is still law but not in the sense the parent appears to be thinking
... except they need to make that transient, formless concept known as money, and governments CAN use that.
