but you have a clue that comes immediately after:
"during build-time
of a program"
Programs are usually not written in a directly executable form - a program would need first to be converted in a form that the specific computer can understand and execute.
as explained here:
Software Build in simpler term is an activity
to translate the human-readable source code into the efficient executable program.
Basically, Build is the process of creating the application program for a software release, by taking all the relevant source code files and compiling them and then creating a build artifacts, such as binaries or executable program, etc.
You can also say that the build process is a combination of several activities which varies for each programming language and for each operating system but please remember the basic concepts are universal.
https://scmquest.com/software-build-knowledge/so a "build-time hash" would be a hash used at the time of "building" the executable form of the program, the point being to prevent any tampering of the executable code, once it has been "built".