Fix per loop iteration in instance

[remyleone]

$ go build -gcflags=all=-d=loopvar=2 ./...                                                                                                                                              
# github.com/scaleway/scaleway-sdk-go/api/instance/v1
api/instance/v1/instance_metadata_sdk.go:38:9: loop variable url now per-iteration, heap-allocated

https://go.dev/wiki/LoopvarExperiment#my-test-fails-with-the-change-how-can-i-debug-it