terraform: share graph walker's variables lock w/ interpolater

The ContextGraphWalker struct includes a lock that's passed down to
BuiltinEvalContext and guards access to interpolation variables as
they're written using SetVariables.

The likely problem being expressed in #5733 is that the same map
reference is also passed down to the Interpolater.Variables field, which
is used for variable lookup.

Here, we plumb the same lock we're using to guard access for writes down
and acquire it before doing variable reads as well. It's not as fine
grained as perhaps it could be, but all the context tests pass and I
believe this should address #5733.

terraform/graph_walk_context.go

@@ -86,6 +86,7 @@ func (w *ContextGraphWalker) EnterPath(path []string) EvalContext {
 			State:         w.Context.state,
 			StateLock:     &w.Context.stateLock,
 			Variables:     variables,
+			VariablesLock: &w.interpolaterVarLock,
 		},
 		InterpolaterVars:    w.interpolaterVars,
 		InterpolaterVarLock: &w.interpolaterVarLock,

terraform/interpolate.go

@@ -28,6 +28,7 @@ type Interpolater struct {
 	State         *State
 	StateLock     *sync.RWMutex
 	Variables     map[string]string
+	VariablesLock *sync.Mutex
 }
 
 // InterpolationScope is the current scope of execution. This is required
@@ -273,6 +274,8 @@ func (i *Interpolater) valueUserVar(
 	n string,
 	v *config.UserVariable,
 	result map[string]ast.Variable) error {
+	i.VariablesLock.Lock()
+	defer i.VariablesLock.Unlock()
 	val, ok := i.Variables[v.Name]
 	if ok {
 		result[n] = ast.Variable{