Remove ytt and jsonnet from binary

[jbrunton]

While it very slightly simplifies setup for some users, there are some significant downsides to bundling ytt and jsonnet into the binary:

• It significantly slows compilation.
• The binary is pretty large.
• It doesn't allow for different versions of ytt and jsonnet.
• The versions used in gflows may be out of date.

Better to simply use them as they are intended.

This would require an easy way to install both as part of a GitHub workflow. The carvel-setup-action will be fine for ytt; from a quick glance, jsonnet may require its own action.

This might also make gflows easier to generalize for other templating tools (e.g. cuelang).