riot-web: 0.15.5 -> 0.15.6 (#42958)

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/riot-web/versions.

These checks were done:

- built on NixOS

- 0 of 0 passed binary check by having a zero exit code.
- 0 of 0 passed binary check by having the new version present in output.
- found 0.15.6 with grep in /nix/store/0gr8hg3lgymk3idlxb9i4q0p6yyq3azz-riot-web-0.15.6
- directory tree listing: https://gist.github.com/5d5d991112bb6e9960e1319112c7c6e9
- du listing: https://gist.github.com/e2ade24d81e7c7b978e65607f2d7b669