Fix `{{..}}` templating

[alexec]

The current templating system appears to be the primary cause of confusion and bugs. These are caused by the following:

1. Template evaluation is unpredicable. By which I mean, you don't know when it'll evaluate and what the output will be. This can result in a workflow that passes just some of the time
2. We use several evaluation technologies: Govaluate, fasttemplate, Expr+Sprig.

Minor issues:

1. {{..}} does not play nicely with Helm templating.
2. podSpecPatch is arguably a work-around rather than a solution.

It would be great to remove this entires class of issue.

To fix this we should:

• Standardize on expr as the templating technology (easy).
• Make evaluation time predicable - at field access time (hard).