detect if running in plan mode and skip envbuilder invocation

[johnstcn]

Currently when running in 'plan' mode we get this warning showing up:

2024-08-01 16:46:21.424+01:00 Plan: 0 to add, 0 to change, 1 to destroy.
2024-08-01 16:46:21.429+01:00 Warning: Cached image not found
2024-08-01 16:46:21.429+01:00 on main.tf line 121, in data "envbuilder_cached_image" "cached":
2024-08-01 16:46:21.430+01:00   121: data "envbuilder_cached_image" "cached" {
2024-08-01 16:46:21.430+01:00 
2024-08-01 16:46:21.431+01:00 get cached image: error probing build cache: uncached RUN command is not supported in cache probe mode
=== ✔ Planning infrastructure [3777ms]

Invoking a cached build in plan mode is likely to be expensive so we should reserve this for apply.
This may involve migrating envbuilder_cached_image to be a resource instead of a data source.

[johnstcn]

Fixed by #15