fix test CI

[hashiblaum]

🛠️ Description

Fixing acceptance test for hcp_vault_plugin resource by removing hard coded variables.

🏗️ Acceptance tests

• Are there any feature flags that are required to use this functionality?
• Have you added an acceptance test for the functionality being added?
• Have you run the acceptance tests on this branch?

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccVaultPlugin'
==> Checking that code complies with gofmt requirements...
golangci-lint run --config ./golangci-config.yml 
TF_ACC=1 go test ./internal/... -v -run=TestAccVaultPlugin -timeout 360m -parallel=10
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/clients    (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/consul     (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/input      (cached) [no tests to run]
=== RUN   TestAccVaultPlugin
--- PASS: TestAccVaultPlugin (925.98s)
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/provider   926.897s

...