Update test to only set CloudProvider on create

* There was an upstream bug that was not persisting the CloudeProvider
  when included in the CreateBuild API body. Now that this fix is in
  place, calling the UpdateBuild with the CloudProvider returns an
  error; thus causing the tests to fail.

Tests Before Change
```
=== RUN   TestAcc_dataSourcePacker
    data_source_packer_image_iteration_test.go:298: unexpected UpdateBuild error, expected nil. Got [PATCH /packer/2021-04-30/organizations/{location.organization_id}/projects/{location.project_id}/builds/{build_id}][409] PackerService_UpdateBuild default  &{Code:6 Details:[] Error:You cannot override a build's Cloud Provider if it has already been set.  Message:You cannot override a build's Cloud Provider if it has already been set. }
    data_source_packer_image_iteration_test.go:466: unexpected CreateChannel error, expected nil. Got [POST /packer/2021-04-30/organizations/{location.organization_id}/projects/{location.project_id}/images/{bucket_slug}/channels][400] PackerService_CreateChannel default  &{Code:9 Details:[] Error:Error creating channel: iteration is incomplete. Message:Error creating channel: iteration is incomplete.}
    data_source_packer_image_iteration_test.go:443: Step 1/1 error: Error running pre-apply refresh: exit status 1

        Error: could not load the channel. something in the request is invalid.

          with data.hcp_packer_image_iteration.alpine,
          on terraform_plugin_test.tf line 4, in data "hcp_packer_image_iteration" "alpine":
           4: 	data "hcp_packer_image_iteration" "alpine" {
```

Tests After Change
```
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-hcp/internal/input	0.700s [no tests to run]
=== RUN   TestAcc_dataSourcePacker
--- PASS: TestAcc_dataSourcePacker (11.98s)
=== RUN   TestAcc_dataSourcePacker_revokedIteration
--- PASS: TestAcc_dataSourcePacker_revokedIteration (7.75s)
=== RUN   TestAcc_dataSourcePackerImage
--- PASS: TestAcc_dataSourcePackerImage (8.06s)
=== RUN   TestAcc_dataSourcePackerIteration
--- PASS: TestAcc_dataSourcePackerIteration (7.77s)
=== RUN   TestAcc_dataSourcePackerIteration_revokedIteration
--- PASS: TestAcc_dataSourcePackerIteration_revokedIteration (7.97s)
PASS
ok  	github.com/hashicorp/terraform-provider-hcp/internal/provider	43.926s
```

internal/provider/data_source_packer_image_iteration_test.go

@@ -278,8 +278,7 @@ func upsertBuild(t *testing.T, bucketSlug, fingerprint, iterationID string) {
 	updateBuildParams.BuildID = build.Payload.Build.ID
 	updateBuildParams.Body = &models.HashicorpCloudPackerUpdateBuildRequest{
 		Updates: &models.HashicorpCloudPackerBuildUpdates{
-			CloudProvider: "aws",
-			Status:        models.HashicorpCloudPackerBuildStatusDONE,
+			Status: models.HashicorpCloudPackerBuildStatusDONE,
 			Images: []*models.HashicorpCloudPackerImageCreateBody{
 				{
 					ImageID: "ami-42",