  Aims. Hard X-ray spectra of black hole binaries (BHB) are produced by
Comptonization of soft seed photons by hot electrons near the black hole. The
slope of the resulting ray spectra is governed by two main parameters: the
electron temperature and the optical depth of the emitting plasma. Given the
extreme brightness of V404~Cyg during the 2015 outburst, we aim to constrain
the source spectral properties using an unprecedented time resolution in hard
X-rays, and to monitor their evolution over the outburst. Methods. We have
extracted and analyzed 602 X-ray spectra of V404 Cyg obtained by the IBIS/ISGRI
instrument on-board INTEGRAL during the 2015 June outburst, using effective
integration times ranging between 8 and 176000 seconds. We have fit the
resulting spectra in the 20-200 keV energy range. Results. We find that while
the light curve and soft X-ray spectra of V404 Cyg are remarkably different
from those of other BHBs, the spectral evolution of V404~Cyg in hard X-rays and
the relations between the spectral parameters are consistent with those
observed in other BHBs. We identify a hard branch} where the electron
temperature is anti-correlated with the hard X-ray flux, and a soft flaring
branch where the relation reverses. In addition, we find that during long X-ray
plateaus detected at intermediate fluxes, the thermal Comptonization models
fail to describe the spectra. However, the statistics improve if we allow the
absorption to vary freely in the fits to these spectra. Conclusions. We
conclude that the hard branch in V404 Cyg is analogous to the canonical hard
state of BHBs. V404~Cyg never seems to enter the canonical soft state, although
the soft flaring branch bears resemblance to the BHB intermediate state and
ultra-luminous state. The X-ray plateaus are likely the result of absorption by
a Compton-thick outflow which reduces the observed flux by a factor of about
10.[abridged]
