Correct IMDS resource ID query parameter

[chlowell]

Specifying an identity by resource ID doesn't work on Azure Container Instances (and maybe some other platforms) because we specify the resource ID with query parameter "mi_res_id". ACI observes only "msi_res_id", which we should prefer anyway because although IMDS observes both parameters, it documents only msi_res_id. Related: Azure/azure-sdk-for-go#22650