Install GRASS GIS first.

# switch to root
su -
 
# replace USERNAME with your non-root username who installed GRASS GIS
if ! echo $PKG_CONFIG_PATH | grep -q /home/USERNAME/usr/local/share/pkgconfig; then
  echo 'export PKG_CONFIG_PATH="$HOME/usr/local/share/pkgconfig:$PKG_CONFIG_PATH"' > ~/.bash_profile
  . ~/.bash_profile
fi
 
echo 'export LD_LIBRARY_PATH="$HOME/usr/local/src/grass/dist.x86_64-pc-linux-gnu/lib:$LD_LIBRARY_PATH"' > ~/.bash_profile
. ~/.bash_profile
 
# install qgis;
# build queuefiles, not packages, to automatically build any dependencies;
# build command line options for qgis to compile GRASS GIS support
sbopkg -k -i "qgis:GRASS_PREFIX8=$(pkg-config --variable=prefix grass)"