Support RNTuple-based EDM4hep output from standalone executables

[kjvbrt]

Switch the standalone Delphes-to-EDM4hep executables from a hardcoded podio::ROOTWriter to podio::makeWriter(), so any of them can write RNTuple-based files by setting PODIO_DEFAULT_WRITE_RNTUPLE=1 instead of the default TTree-based file, with no other usage change.

Link podio::podioIO instead of podio::podioRootIO since makeWriter() lives in libpodioIO.so, and document the new env var in each executable's --help text.

[jmcarcell]

It would be better if an argument could be passed to the executables for choosing rntuple since makeWriter takes it (for example in https://github.com/AIDASoft/podio/blob/master/tests/root_io/write_interface_rntuple.cpp#L4). I say this because an env variable can a bit sneaky and one may not get the behaviour that one wants. Basically by passing "default" to makeWriter the env variable can be used but a switch to override whatever the value of the env variable may be good (then the command doesn't depend on the environment).

[kjvbrt]

Good point, added a --rntuple flag that gets passed through to podio::makeWriter(); the env var still works as a fallback default.

[kjvbrt]

Note there's currently no --root flag to force TTree explicitly, so if PODIO_DEFAULT_WRITE_RNTUPLE is set globally, the only way to force TTree per-run right now is to unset that env var for the invocation. Maybe not really needed :)