Fix flaky tests when legacyplatform analyzer is invoked

[xnyo]

Fixes flaky tests involving the legacyplatform analyzer (angular) when https://grafana.com/api/plugins/angular_patterns is called too quickly:

--- FAIL: TestRunner (0.39s)
    --- FAIL: TestRunner/AllFilesPresentButEmpty (0.23s)
        runner_test.go:72: legacyplatform: json: cannot unmarshal object into Go value of type []legacyplatform.gcomPattern
FAIL
FAIL    github.com/grafana/plugin-validator/pkg/runner  0.488s

This is happening because calling the API too fast will trigger the rate limit in GCOM:

{
  "code": "TooManyRequests",
  "message": "Rate limit exceeded.",
  "requestId": "d81fb3f1-ed30-4e66-8da6-4dc7fb7a2d38"
}

With this PR, we cache the result and call the GCOM API only once

[andresmgot]

Do we need to expire the cache?

[xnyo]

Do we need to expire the cache?

No, those hardly ever change and it this only lives in memory anyways, so re-running the plugin-validator will expire the cache.

We only need it so we don't hit GCOM API too quickly and get back a 429, mostly when running tests

[andresmgot]

LGTM