backend/local: do not prompt for input vars twice

backend, which resulted in users being prompted for input variables
twice. This PR fixes that by setting AllowUnsetVariables to true for the
state context.

Fixes #26027

backend/local/backend_local.go

@@ -114,11 +114,14 @@ func (b *Local) context(op *backend.Operation) (*terraform.Context, *configload.
 		// diags from here will be caught above
 
 	} else {
+		// WHOOOPS
 		log.Printf("[TRACE] backend/local: building context for current working directory")
 		tfCtx, configSnap, ctxDiags = b.contextDirect(op, opts)
 
 		// create a validation context with no state
-		validateCtx, _, _ = b.contextDirect(op, validateOpts)
+		validateOp := op
+		validateOp.AllowUnsetVariables = true
+		validateCtx, _, _ = b.contextDirect(validateOp, validateOpts)
 		// diags from here will be caught above
 	}
 	diags = diags.Append(ctxDiags)