fix(federation): assignment GraphqlSchemaHost#schema in factory

[randomprofilename]

GraphqlSchemaHost#schema wasn't assigned after GraphqlFederationModule initialization

resolves #1478

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #1478

What is the new behavior?

GraphqlSchemaHost#schema assigned and can be used correctly after GraphqlFederationModule initialization

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[randomprofilename]

I haven't added or updated any doc - working with GraphQLSchemaHost is already described here https://docs.nestjs.com/graphql/quick-start#accessing-generated-schema

Please tell me if i should do smth else to finish PR, thx :)

[kamilmysliwiec]

LGTM