AgdaStdlib: 0.14 -> 0.15

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 0.15 with grep in /nix/store/6787gs55hpa46z4507ji4jz5za51lyqy-agda-stdlib-0.15
- directory tree listing: https://gist.github.com/386f03ce4900379ef75d5d01bcc2cebb