use defined expiration var cacheExpiration when setting transient

[circlecube]

Proposed changes

Currently, there is a protected class variable cacheExpiration. This value is not used anywhere. It should be used when saving the release payload as transient. Instead the transient is set with the repeated value of 6 hours.

This PR uses the value that is in place already as it appears to be intended for. So, now if someone sets the expiration it will actually be used.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [] Documentation Update (if none of the other choices apply)

Checklist

• [] Linting and tests pass locally with my changes
• [] I have added tests that prove my fix is effective or that my feature works
• [] I have added necessary documentation (if appropriate)

Further comments