Go: Add `vendor` directory to `mixed-layout` test with `go.work` file

The `go.work` file here should not get discovered by the autobuilder

go/ql/integration-tests/mixed-layout/src/module/vendor/example.com/test/add.go

@@ -0,0 +1,5 @@
+package test
+
+func Add(a, b int) int {
+	return a + b
+}

go/ql/integration-tests/mixed-layout/src/module/vendor/example.com/test/go.work

@@ -0,0 +1,3 @@
+go 1.22.0
+
+use .

go/ql/integration-tests/mixed-layout/src/module/vendor/modules.txt

@@ -0,0 +1,3 @@
+# example.com/test v0.1.0
+## explicit; go 1.14
+example.com/test

go/ql/integration-tests/mixed-layout/src/workspace/go.work.sum

@@ -0,0 +1,3 @@
+golang.org/x/crypto v0.21.0/go.mod h1:0BP7YvVV9gBbVKyeTG0Gyn+gZm94bibOW5BjDEYAOMs=
+golang.org/x/term v0.18.0/go.mod h1:ILwASektA3OnRv7amZ1xhE/KTR+u50pbXfZ03+6Nx58=
+golang.org/x/text v0.14.0/go.mod h1:18ZOQIKpY8NJVqYksKHtTdi31H5itFRjB5/qKTNYzSU=