command/build: Add warn-on-undeclared-var flag

Now that the default behaviour is to not display warnings for undeclared variables
an optional flag has been added to toggle the old behaviour.

```
~>  go run . build -warn-on-undeclared-var -var-file command/test-fixtures/validate/var-file-tests/undeclared.pkrvars.hcl command/test-fixtures/validate/var-file-tests/basic.pkr.hcl
Warning: Undefined variable

The variable "unused" was set but was not declared as an input variable.
To declare variable "unused" place this block in one of your .pkr.hcl files,
such as variables.pkr.hcl

variable "unused" {
  type    = string
  default = null

}

file.chocolate: output will be in this color.

Build 'file.chocolate' finished after 762 microseconds.

==> Wait completed after 799 microseconds

==> Builds finished. The artifacts of successful builds are:
--> file.chocolate: Stored file: chocolate.txt
```

command/build.go

@@ -393,7 +393,8 @@ Options:
   -parallel-builds=1            Number of builds to run in parallel. 1 disables parallelization. 0 means no limit (Default: 0)
   -timestamp-ui                 Enable prefixing of each ui output with an RFC3339 timestamp.
   -var 'key=value'              Variable for templates, can be used multiple times.
-  -var-file=path                JSON or HCL2 file containing user variables.
+  -var-file=path                JSON or HCL2 file containing user variables, can be used multiple times.
+  -warn-on-undeclared-var       Display warnings for user variable files containing undeclared variables.
 `
 
 	return strings.TrimSpace(helpText)

command/cli.go

@@ -86,6 +86,7 @@ func (ba *BuildArgs) AddFlagSets(flags *flag.FlagSet) {
 	flagOnError := enumflag.New(&ba.OnError, "cleanup", "abort", "ask", "run-cleanup-provisioner")
 	flags.Var(flagOnError, "on-error", "")
 
+	flags.BoolVar(&ba.MetaArgs.WarnOnUndeclaredVar, "warn-on-undeclared-var", false, "Show warnings for variable files containing undeclared variables.")
 	ba.MetaArgs.AddFlagSets(flags)
 }