Go allows you to override global variables at build time during the linker stage:

$ go build -o /tmp/prog-custom -ldflags "-X main.overrideableValuePhrase=123456" ./cmd/prog

Requirements:

• It must be a global variable and not a constant.
• It might be in the package that you are telling it to build. This definitely works for executables and, with the removal of support for binary-only packages (BOPs), it probably doesn’t apply whatsoever to intermediate packages.
• It must be a string (so, process it from the init() function).

It doesn’t matter whether it is an exported or unexported symbol.