ci: add GitHub Actions cron (#16)

* ci: add dependabot

* ci: revamp logic

Merge username and password as a single auth token. It doesn't make
sense to split out the user and password since they are so tied
together. Might as well treat the whole think as a secret blob.

Remove Travis-CI. Travis is dead for OSS.

Add GitHub Actions cron. Remove cachix as it's pushing too much stuff.

Merge all of the CI logic into a single ci.sh script.

dockerhub-metadata

@@ -6,10 +6,8 @@
 # Usage: ./dockerhub-metadata <user> <password> [org]
 set -euo pipefail
 
-username=$1
-password=$2
-org=${3:-nixpkgs}
-user=$username:$password
+user=$1
+org=${2:-nixpkgs}
 
 nix_eval() {
   nix-instantiate --strict --eval --json "$@"