Well, then
- Travis cache is out of question:
- It cannot pass information between stages unless the two jobs have exactly the same configuration (i.e. correspond 1-1)
- Using any 3rd-party service is impossible due to conflicting requirements:
- You require build agents to be authenticated to be able to contribute data, but you refuse to provide them with any means to authenticate themselves because they are untrusted.
The only idea I have is to use the travis console utility to extract information about the current build, locate the appropriate jobs within it and extract the necessary data from their build logs (a build log is the only build result that Travis retains).