Include esy fallback for unsupported platforms

[jchavarri]

@aantron added in bisect_ppx a really nice fallback approach where after downloading, the ppx will build with esy if the user platform is not supported by the prebuilt binaries.

Would be nice to add this to the hello-ppx-esy example.

[aantron]

Bisect_ppx is set up so that this fallback approach is actually the primary build system for release binaries. Since the binaries are missing in master, this triggers the "fallback" when master is installed in CI, and the CI caches the result of the binary build to create the binary release (branch binaries).