runtime: implement internal/godebug.setUpdate

This function was a stub, but it really needs to be implemented for full
Go 1.20 support. Without it, the archive/zip tests will fail.

src/runtime/env_unix.go

@@ -11,6 +11,12 @@ func syscallSetenv(key, value string) {
 	valdata := cstring(value)
 	// ignore any errors
 	libc_setenv(&keydata[0], &valdata[0], 1)
+	if key == "GODEBUG" && godebugUpdate != nil {
+		// Starting with Go 1.20, we need to call a callback (set by
+		// internal/godebug) to notify the GODEBUG environment variable has
+		// changed. This is necessary to get archive/zip to pass tests.
+		godebugUpdate(key, value)
+	}
 }
 
 // Update the C environment if cgo is loaded.

src/runtime/runtime.go

@@ -96,8 +96,11 @@ func SetFinalizer(obj interface{}, finalizer interface{}) {
 	// Unimplemented.
 }
 
+var godebugUpdate func(string, string)
+
 //go:linkname godebug_setUpdate internal/godebug.setUpdate
 func godebug_setUpdate(update func(string, string)) {
-	// Unimplemented. The 'update' function needs to be called whenever the
-	// GODEBUG environment variable changes (for example, via os.Setenv).
+	// The 'update' function needs to be called whenever the GODEBUG environment
+	// variable changes (for example, via os.Setenv).
+	godebugUpdate = update
 }