For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:Branch_devel_tools/openSUSE_Tumbleweed/home:ojkastl_buildservice:Branch_devel_tools.repo
zypper refresh
zypper install nix-installer
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:Branch_devel_tools/15.6/home:ojkastl_buildservice:Branch_devel_tools.repo
zypper refresh
zypper install nix-installer