hcl2: don't show an empty source on var errors

If a variable is defined and overriden in the packer command-line, but
there's a problem during the evaluation of this override (type error
typically), we show an error message which details the problem.

This message points to a temporary in-memory HCL2 "file" that we use for
parsing and evaluating the expression for the variable, but since it's
virtual, there's no point in using this as the source for the error, as
it will always yield "line 0" and no contents.

So, in order to limit confusion here, we remove the source for this
error message.

hcl2template/types.variables.go

@@ -741,7 +741,6 @@ func (cfg *PackerConfig) collectInputVariableValues(env []string, files []*hcl.F
 					Severity: hcl.DiagError,
 					Summary:  "Invalid argument value for -var variable",
 					Detail:   fmt.Sprintf("The received arg value for %s is not compatible with the variable's type constraint: %s.", name, err),
-					Subject:  expr.Range().Ptr(),
 				})
 				val = cty.DynamicVal
 			}