Fix Stacks vcs-repo encoding

[brandonc]

Description

There was a regression in terraform-provider-tfe after the last go-tfe bump which I narrowed down to this encoding change.

Testing plan

See test output below

Output from tests

Including output from tests may require access to a TFE instance. Ignore this section if you have no environment to test against.

$ OAUTH_CLIENT_GITHUB_TOKEN=$GITHUB_TOKEN ENABLE_BETA=1 go test ./... -v -run "TestWorkspacesCreateTableDriven/when_options_include_vcs-repo"
?       github.com/hashicorp/go-tfe/examples/backing_data       [no test files]
?       github.com/hashicorp/go-tfe/examples/configuration_versions     [no test files]
?       github.com/hashicorp/go-tfe/examples/registry_modules   [no test files]
?       github.com/hashicorp/go-tfe/examples/organizations      [no test files]
?       github.com/hashicorp/go-tfe/examples/run_errors [no test files]
?       github.com/hashicorp/go-tfe/examples/state_versions     [no test files]
?       github.com/hashicorp/go-tfe/examples/users      [no test files]
?       github.com/hashicorp/go-tfe/examples/workspaces [no test files]
?       github.com/hashicorp/go-tfe/mocks       [no test files]
=== RUN   TestWorkspacesCreateTableDriven
=== RUN   TestWorkspacesCreateTableDriven/when_options_include_vcs-repo
--- PASS: TestWorkspacesCreateTableDriven (8.21s)
    --- PASS: TestWorkspacesCreateTableDriven/when_options_include_vcs-repo (4.75s)
=== RUN   TestWorkspacesCreateTableDrivenWithGithubApp
    workspace_integration_test.go:476: Export a valid GITHUB_APP_INSTALLATION_ID before running this test!
--- SKIP: TestWorkspacesCreateTableDrivenWithGithubApp (0.00s)
PASS
ok      github.com/hashicorp/go-tfe     8.470s

$ OAUTH_CLIENT_GITHUB_TOKEN=$GITHUB_TOKEN ENABLE_BETA=1 go test ./... -v -run "TestStackReadUpdateDelete"
?       github.com/hashicorp/go-tfe/examples/backing_data       [no test files]
?       github.com/hashicorp/go-tfe/examples/configuration_versions     [no test files]
?       github.com/hashicorp/go-tfe/examples/organizations      [no test files]
?       github.com/hashicorp/go-tfe/examples/registry_modules   [no test files]
?       github.com/hashicorp/go-tfe/examples/run_errors [no test files]
?       github.com/hashicorp/go-tfe/examples/state_versions     [no test files]
?       github.com/hashicorp/go-tfe/examples/users      [no test files]
?       github.com/hashicorp/go-tfe/examples/workspaces [no test files]
?       github.com/hashicorp/go-tfe/mocks       [no test files]
=== RUN   TestStackReadUpdateDelete
--- PASS: TestStackReadUpdateDelete (6.50s)
PASS
ok      github.com/hashicorp/go-tfe     6.773s

[ctrombley]

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Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.