the -check-variables option has been removed from 0.12

Now the validator behaves as if this were `false`.
See: hashicorp/terraform#21408 (comment)

However, as a result of my investigation, the validator uses the source
module to check whether the syntax is valid and types/values are being
used correctly.

To react with the changes, this commit adds PWD=directory as an
temporary environment variable on the command execution, because validate
command seems to refer to the current directory.

test/make.sh

@@ -94,6 +94,9 @@ function check_terraform() {
   local rval=125
   # fmt is before validate for faster feedback, validate requires terraform
   # init which takes time.
+  echo "Running terraform validate"
+  #shellcheck disable=SC2156
+  find . -name "*.tf" -exec bash -c 'PWD=$(dirname "{}") terraform validate' \;
   echo "Running terraform fmt"
   find_files . -name "*.tf" -print0 \
     | compat_xargs -0 -n1 dirname \