terraform: Propagate apply-time variables from plan to apply

We need to remember which ephemeral values were set during planning so that
we can require them to be set again (possibly to a different value) during
the apply step.

internal/terraform/context_apply.go

@@ -299,6 +299,15 @@ func (c *Context) applyGraph(plan *plans.Plan, config *configs.Config, opts *App
 			SourceType: ValueFromPlan,
 		}
 	}
+	// Apply-time variables need to be merged in too.
+	// FIXME: We should check that all of these match declared variables and
+	// that all of them are declared as ephemeral, because all non-ephemeral
+	// variables are supposed to come exclusively from plan.VariableValues.
+	if opts != nil {
+		for n, vv := range opts.SetVariables {
+			variables[n] = vv
+		}
+	}
 	if diags.HasErrors() {
 		return nil, walkApply, diags
 	}

internal/terraform/context_plan.go

@@ -324,6 +324,7 @@ The -target option is not for routine use, and is provided only for exceptional
 		if len(varMarks) > 0 {
 			plan.VariableMarks = varMarks
 		}
+		plan.ApplyTimeVariables = applyTimeVariables
 		plan.TargetAddrs = opts.Targets
 	} else if !diags.HasErrors() {
 		panic("nil plan but no errors")