fix: read the space id from the plan rather than the state for a variable

[benPearce1]

fixes #775

before:

# octopusdeploy_variable.string will be updated in-place
  ~ resource "octopusdeploy_variable" "string" {
      + encrypted_value = (known after apply)
        id              = "5a728184-0d52-4039-9897-58a7f6d63791"
      + key_fingerprint = (known after apply)
        name            = "stringvar"
      + space_id        = "Spaces-962"
        # (6 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.
octopusdeploy_variable.string: Modifying... [id=5a728184-0d52-4039-9897-58a7f6d63791]
╷
│ Error: update variable failed
│ 
│   with octopusdeploy_variable.string,
│   on main.tf line 31, in resource "octopusdeploy_variable" "string":
│   31: resource "octopusdeploy_variable" "string" {
│ 
│ Octopus API error: Resource is not found or it doesn't exist in the current space context. Please contact your administrator for more information. [] 
╵

after:

Terraform will perform the following actions:

  # octopusdeploy_variable.string will be updated in-place
  ~ resource "octopusdeploy_variable" "string" {
      + encrypted_value = (known after apply)
        id              = "11f2295a-2f3b-428c-9dc8-36cf911088e4"
      + key_fingerprint = (known after apply)
        name            = "stringvar"
      + space_id        = "Spaces-963"
        # (6 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.
octopusdeploy_variable.string: Modifying... [id=11f2295a-2f3b-428c-9dc8-36cf911088e4]
octopusdeploy_variable.string: Modifications complete after 0s [id=11f2295a-2f3b-428c-9dc8-36cf911088e4]

Apply complete! Resources: 0 added, 1 changed, 0 destroyed.

[HuyPhanNguyen]

LGTM