test(build): update TestPipeReader call to new splitReader(ctx, reader) signature

The earlier commit on this branch added context.Context as the first
argument of splitReader so the copy goroutine can unblock on ctx.Done().
TestPipeReader still passed only the reader, which tripped the lint/CI
typecheck:

    pkg/backend/build/builder_test.go:287:24: not enough arguments in call to splitReader
        have (*strings.Reader)
        want (context.Context, io.Reader)

Pass context.Background() so the test exercises the new signature.
`context` is already imported by other tests in this file.

Author: SAY-5

pkg/backend/build/builder_test.go

@@ -284,7 +284,7 @@ func TestBuilderSuite(t *testing.T) {
 
 func TestPipeReader(t *testing.T) {
 	r := strings.NewReader("some io.Reader stream to be read\n")
-	r1, r2 := splitReader(r)
+	r1, r2 := splitReader(context.Background(), r)
 	var wg sync.WaitGroup
 	wg.Add(1)
 	go func() {