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