core: Unmark provisioner config before validation

[alisdair]

Sensitive values in provisioner configuration would cause errors in the validate phase. We need to unmark these value before serializing the config value for the provisioner plugin.

Fixes #27763. Also separately fixed on the main branch.

[codecov]

Codecov Report

Merging #27819 (6fb010f) into v0.14 (74134a8) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
terraform/eval_validate.go	59.14% <100.00%> (+0.25%)	⬆️
terraform/node_resource_plan.go	95.32% <0.00%> (-1.87%)	⬇️
dag/walk.go	91.60% <0.00%> (-0.70%)	⬇️
terraform/evaluate.go	52.89% <0.00%> (-0.42%)	⬇️

[ghost]

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