For anyone looking for a simple commandline solution, the following shell command will also do it. It requires the Github CLI + JQ:
gh api -H 'Accept: application/vnd.github+json' /repos/realm/realm-kotlin/actions/caches --paginate | jq -r '.actions_caches | .[].key' | sed -e 's/|/%7c/g' -e 's/\[/%5b/g' -e 's/\]/%5d/g' | xargs -I {} sh -c 'gh api --method DELETE -H "Accept: application/vnd.github+json" /repos/<OWNER>/<REPO>/actions/caches?key={} --silent'
In this case, I'm using sed to URL encode special characters in the key. I just needed to handle |, [ and ]. Others can be added as needed. See e.g. https://gist.github.com/jaytaylor/5a90c49e0976aadfe0726a847ce58736#file-url_encode-sh-L11
