octave: add runtime texinfo dependency

makeinfo (provided by the texinfo package) is used by the "help"
command in Octave to display info about functions, etc.  By default,
Octave looks for "makeinfo" in the PATH, rather than specifying the
location of the executable.  This results in Nix not being aware that
makeinfo is required at runtime (so unless you happen have makeinfo
available from your path, "help" won't work).

This patch fixes that by setting the path to makeinfo in Octave,
thereby creating a runtime dependency on texinfo.