`generate-installer` refuses a macOS archive, so the last step of the
pipeline cannot succeed there. Left to fail on its own it does so only
after a full source build has been spent, so check the platform up front,
like the entry name guards above it.
Packaging a version by hand means running `pyenv install`, `pyenv binary
save` and `pyenv binary generate-installer` in turn and keeping the names
they produce in step. `build` runs the three in one pass, so the archive,
its metadata and the definition all come out named after the entry.
* The entry name is given explicitly as `<version>:<entry>`, reusing the
syntax `pyenv install` already accepts, so a packaged build can sit
beside a plain source install of the same version
* Entry names that `pyenv install` would not install under (`latest`), or
that would point outside versions/, are refused before anything is
compiled rather than after
* Linux only for now, matching `save` and `generate-installer`
