jenkins: Set BUILD_USER for Debian and Snap builds

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3971

jenkins/build-linux.bash

@@ -42,6 +42,7 @@ for plat in "${platforms[@]}"; do
 	echo
 done
 
+export BUILD_USER=deb
 go run build.go -goarch amd64 deb
 go run build.go -goarch i386 deb
 go run build.go -goarch armel deb
@@ -50,6 +51,7 @@ go run build.go -goarch arm64 deb
 
 mv *.deb "$WORKSPACE"
 
+export BUILD_USER=snap
 go run build.go -goarch amd64 snap
 go run build.go -goarch armhf snap
 go run build.go -goarch arm64 snap