Make sample check configurable to use repository mirrors

[ljacomet]

When testing samples, there is no mirroring of repositories possible AFAICT. This is causing noise in CI due to the Bintray availability issues.
sample-check should be enhanced so that it can run with an option similar to GradleExecuter.withRepositoryMirrors() to reduce flakiness in execution.

[wolfs]

It is now possible to customize the invocation from the runner: #27

That should be enough to implement this issue, right?

[eriwen]

This is now possible using SampleModifiers after a1e9ad6. This is included in v0.6.0.

See the docs here: https://github.com/gradle/exemplar/#common-sample-modification