Maybe the fresh install isn’t necessary. I was able to build Ctrlr 6.0. The one that is in the master branch at the Github page is version 6.04 if I’m not mistaken. Go to the stable branch and try the commit from January 10th, 2020. It even says “Linux project update”. What more do you want? If you want to download version 5.3.201 you have to go back to March 2016. I’ll have to search for the right date.
Are people building VST’s for Linux? Depends on how many people actually use Linux, right? But I’m afraid that the majority uses Windows or macOS. It is possible to run Windows VST’s through Wine or in certain Linux Daws like Carla or Reaper. There are even wrappers to run Windows VST’s on Linux. So I guess the answer to your question is: There is no need to build VST’s specific for Linux.
Ok, Linux uses his own VST format, LV2. But I don’t think big companies want to spend time on building an LV2 just because Linux has a much smaller user base and therefore it’s not commercially interesting. Juce doesn’t support LV2. Since Ctrlr is based on Juce, Ctrlr doesn’t support it as well. Unfortunately.