In pipelines that have many environments and many CI variables, the
decryption in `Ci::HasVariable#to_runner_variable` is a
significant bottleneck due to repeated decryption via
`OpenSSL::PKCS5.pbkdf2_hmac`. We can optimize this by caching the
value in the request. In one customer's pipeline, this cuts down
`Ci::CreatePipelineService` from 80+ seconds to 30 seconds.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/326271

Changelog: performance