*: git.monogon.dev -> source.monogon.dev

This implements T882, setting our (virtual) GOPATH to source.monogon.dev
for this repository.

Test Plan: Refactor, CI only.

X-Origin-Diff: phab/D686
GitOrigin-RevId: c5e2309089948ffc3a98e68e2e0e1cbb157d3a36
diff --git a/build/fietsje/shelf.go b/build/fietsje/shelf.go
index d3a1a1a..ea709be 100644
--- a/build/fietsje/shelf.go
+++ b/build/fietsje/shelf.go
@@ -26,7 +26,7 @@
 
 	"github.com/golang/protobuf/proto"
 
-	pb "git.monogon.dev/source/nexantic.git/build/fietsje/proto"
+	pb "source.monogon.dev/build/fietsje/proto"
 )
 
 // The Shelf is a combined cache and dependency lockfile, not unlike go.sum. It's implemented as a text proto file on