Update the buildflags cty code to handle unknown values. When hcl
decodes a value with an invalid variable name, it appends a diagnostic
for the error and then returns an unknown value so it can continue
processing the file and finding more errors.
The iteration code has now been changed to use a rangefunc from go 1.23
and it skips empty or unknown values. Empty values are valid when they
are skipped and unknown values will have a diagnostic for itself.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
