gradle build triggers signArchives

[ghost]

Describe the bug
gradle build task triggers signArchives, it causes to not be able to build synapse in local.

To Reproduce
Steps to reproduce the behavior:

1. ./gradlew build

Outcome:

* What went wrong:

Execution failed for task ':synapse-aws-auth:signArchives'.

> Cannot perform signing task ':synapse-aws-auth:signArchives' because it has no configured signatory

Expected behavior
It should be able to build de library and create the artefacts for local use. We expect to have a separated task for signArchives which it's only used by library maintainers.