[BUG] Validator throws "attribute .servers.invalid url" error

Description

When validating a remote OpenAPI spec with a server URL that uses variable substitution placeholders, the openapi-generator throws the following error:

❯ java -jar openapi-generator-cli.jar validate -i https://gist.githubusercontent.com/badsyntax/50ba5ec813a6ecac942b6ee177de0cae/raw/6586e44937cd91e99e1344cdda4b4708cda76b98/test.yml
Validating spec (https://gist.githubusercontent.com/badsyntax/50ba5ec813a6ecac942b6ee177de0cae/raw/6586e44937cd91e99e1344cdda4b4708cda76b98/test.yml)
Errors:
        - attribute .servers.invalid url : https://{host}/admin

[error] Spec has 1 errors.

If you validate the same file from local disk, no errors are thrown:

❯ java -jar openapi-generator-cli.jar validate -i ./test.yml 
Validating spec (./test.yml)
No validation issues detected.

openapi-generator version

5.0.0-beta3

OpenAPI declaration file content or url

https://gist.githubusercontent.com/badsyntax/50ba5ec813a6ecac942b6ee177de0cae/raw/6586e44937cd91e99e1344cdda4b4708cda76b98/test.yml

openapi: 3.0.3
info:
  title: Title
  version: 0.51.0
servers:
  - url: https://{host}/admin
    variables:
      host:
        default: box.example.com
        description: The API hostname.
paths:
  /me:
    get:
      operationId: getMe
      responses:
        200:
          description: Successful operation

Generation Details

Steps to reproduce

Shows the bug when validating a remote file:

java -jar openapi-generator-cli.jar validate -i https://gist.githubusercontent.com/badsyntax/50ba5ec813a6ecac942b6ee177de0cae/raw/6586e44937cd91e99e1344cdda4b4708cda76b98/test.yml

Show no bug when validating a local file:

java -jar openapi-generator-cli.jar validate -i ./test.yml

Related issues/PRs

Possibly related: https://github.com/swagger-api/swagger-parser/issues/1329

Suggest a fix

Start with the CLI validate command and compare how it loads remote and local OpenAPI documents. Reproduce using the provided remote URL and test.yml, then confirm that a server URL using {host} with a declared default validates identically from both sources without the reported error.