In this build I tried adding the addons / pkg key at the top level of my .travis.yml, which also had all 4 OSs listed at the top level. Apparently this caused the Linux and macOS jobs to hang at a password prompt (probably due to an internal su command) inside a “Configuring default pkg options” step, while the Windows job got lucky.
Looks like the pkg addon doesn’t know to run itself only on FreeBSD, unlike the brew (macOS only) and apt (Linux only) addons. I’m working around with a jobs / include list with OS-specific sections in the next build.