fix: global ajv cache

[Eomm]

I faced this issue when I was using this plugin in tests:

fastify.register(fastifyEnv, {
      confKey: 'secrets',
      data: opts.configData,
      schema: fastify.getSchema('schema:dotenv')
    })

This plugin has a global ajv instance that is caching the schemas.
This cache trigger this error:

Subtest: the application should start
    not ok 1 - schema with key or id "schema:dotenv" already exists
      ---
      stack: |
        Ajv._checkUnique (node_modules/env-schema/node_modules/ajv/lib/core.ts:721:13)

This change turns off the cache because the schema provided to the env-schema is single and it does not require the cache used by ajv.

https://ajv.js.org/options.html#addusedschema

[darkgl0w]

LGTM

[mcollina]

lgtm