Make the --matrix flag value be YAML

[ajschmidt8]

Right now the --matrix flag accepts ; delimited key/value pairs like this: --matrix "cuda=11.5;arch=$(arch)"

We should switch the --matrix value to be YAML to be consistent with dependencies.yaml.

e.g. --matrix '{"cuda": "11.6", "arch": "x86_64"}

[ajschmidt8]

Since this would be yet another breaking change, it might make sense to tackle #12 first.