Properly respect environment variable on checkpoint setup

The checkpoint update missed the environment variable check for
disabling the checks so this adds in the check and properly
disables checks when requested.

lib/vagrant/util/checkpoint_client.rb

@@ -39,6 +39,10 @@ module Vagrant
         rescue LoadError
           @logger.warn("checkpoint library not found. disabling.")
         end
+        if ENV["VAGRANT_CHECKPOINT_DISABLE"]
+          @logger.debug("checkpoint disabled via explicit user request")
+          @enabled = false
+        end
         @files = {
           signature: env.data_dir.join("checkpoint_signature"),
           cache: env.data_dir.join("checkpoint_cache")

test/unit/vagrant/util/checkpoint_client_test.rb

@@ -23,7 +23,8 @@ describe Vagrant::Util::CheckpointClient do
   end
 
   describe "#setup" do
-    before{ subject.setup(env) }
+    let(:environment){ {} }
+    before{ with_temp_env(environment){ subject.setup(env) } }
 
     it "should enable after setup" do
       expect(subject.enabled).to be(true)
@@ -32,6 +33,14 @@ describe Vagrant::Util::CheckpointClient do
     it "should generate required paths" do
       expect(subject.files).not_to be_empty
     end
+
+    context "with VAGRANT_CHECKPOINT_DISABLE set" do
+      let(:environment){ {"VAGRANT_CHECKPOINT_DISABLE" => "1"} }
+
+      it "should not be enabled after setup" do
+        expect(subject.enabled).to be(false)
+      end
+    end
   end
 
   describe "#check" do