Improve reliability of NO_COLOR tests.

Use `env` to ensure that `NO_COLOR` is set to the desired value (or
unset) regardless of any initialization that may occur when `script`
forks a shell to run jq in.

tests/shtest

@@ -573,30 +573,29 @@ test_no_color=true
 $msys && test_no_color=false
 $mingw && test_no_color=false
 if $test_no_color && command -v script >/dev/null 2>&1; then
- unset NO_COLOR
  if script -qc echo /dev/null >/dev/null 2>&1; then
  faketty() { script -qec "$*" /dev/null; }
  else # macOS
  faketty() { script -q /dev/null "$@" /dev/null |
  sed 's/^\x5E\x44\x08\x08//'; }
  fi
 
- faketty $JQ_NO_B -n . > $d/color
+ faketty env -u NO_COLOR $JQ_NO_B -n . > $d/color
  printf '\033[0;90mnull\033[0m\r\n' > $d/expect
  od -tc $d/expect
  od -tc $d/color
  cmp $d/color $d/expect
- NO_COLOR= faketty $JQ_NO_B -n . > $d/color
+ faketty env NO_COLOR= $JQ_NO_B -n . > $d/color
  printf '\033[0;90mnull\033[0m\r\n' > $d/expect
  od -tc $d/expect
  od -tc $d/color
  cmp $d/color $d/expect
- NO_COLOR=1 faketty $JQ_NO_B -n . > $d/color
+ faketty env NO_COLOR=1 $JQ_NO_B -n . > $d/color
  printf 'null\r\n' > $d/expect
  od -tc $d/expect
  od -tc $d/color
  cmp $d/color $d/expect
- NO_COLOR=1 faketty $JQ_NO_B -Cn . > $d/color
+ faketty env NO_COLOR=1 $JQ_NO_B -Cn . > $d/color
  printf '\033[0;90mnull\033[0m\r\n' > $d/expect
  od -tc $d/expect
  od -tc $d/color