Creating an environment variable with windows in a matrix causes MSBuild.exe to hang forever

I have a .Travis.yml file that starts a Linux build and a Windows build in a matrix. The windows build has been simplified so that it adds the build tools to the PATH, calls MSBuild.exe with no arguments, and ends. Without any environment variables set, it prints an error message and exits. With one set, it hangs for 10 minutes with no output after calling msbuild and gets killed. What do I do?

I’m pretty sure it only happens if the environment variable is a secret, also.

NVM, is a duplicate of Secrets-Are-Killing-Windows.