This will ensure pre Go 1.16 behavior is maintained once Debian migrates to Go 1.16.
piuparts will fail to build with Go 1.16 otherwise [1].
Here is the relevant bit of Go 1.16 changelog [2].
Another option here would be turning the relevant (go) helper into a module. Given its size and context here, I thought it would be better to just set the environment variable in question. Let me know if there are any issues with the current approach.
[1] https://launchpad.net/ubuntu/+source/piuparts/1.1.3/+build/21477124 [2] https://golang.org/doc/go1.16#go-command