Refactor the `strict` boolean passed into evaluation at the top-level to
be a (two-variant, so far) EvalMode enum of Lazy and Strict.
This is more explicit than a boolean, and if we ever add more EvalModes
it's a simple extension of the enum.
Change-Id: I3de50e74ec971011664f6cd0999d08b792118410
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12186
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: aspen <root@gws.fyi>
b7a6fc2812