Unfortunately, GitHub does not support searches in the wiki. It does, however, support cloning the wiki as a git repo! Add
git grep and you've got a stew going.
GitHub wiki pages have a "Clone URL" button to copy the right URL
$ git clone https://github.com/me/my-project.wiki my-project-wiki
Cloning into 'my-project-wiki'...
remote: Counting objects: 3924, done.
remote: Compressing objects: 100% (3905/3905), done.
remote: Total 3924 (delta 2388), reused 0 (delta 0)
Receiving objects: 100% (3924/3924), 510.08 KiB | 104 KiB/s, done.
Resolving deltas: 100% (2388/2388), done.
(devmac-12) (11:40:16) ~/projects 1.8.7
$ cd my-project-wiki
(devmac-12) (11:41:57) ~/projects/my-project-wiki [master] 1.8.7
$ git grep "something to search for"
You'll get the results piped to your pager.