Fix eventual consistency issue with github_actions_environment_variable

[marcoths]

Resolves #2047

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Before the change?

• When creating github_actions_environment_variable resources, there is the chance on which the variable gets created, but the read operation fails - for example due to eventual consistency. Before this change, Terraform would issue the error

Error: Provider produced inconsistent result after apply

When applying changes to
github_actions_environment_variable.my_variable, provider
"provider[\"registry.terraform.io/integrations/github\"]" produced an
unexpected new value: Root object was present, but now absent.

This is a bug in the provider, which should be reported in the provider's own
issue tracker.

After the change?

• The retry package in the terraform plugin sdk has a struct called StateChangeConf which implements a retry mechanism for these cases. This is an example from hashicorp/terraform-provider-aws

Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

No breaking change

Please see our docs on breaking changes to help!

• [ ] Yes
• [x] No

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