is_command stdlib function bug fix (#262)

src/std/main.ab

@@ -130,10 +130,10 @@ pub fun includes(arr, value) {
 }
 
 pub fun is_command(command: Text): Bool {
-    if (unsafe $command -v "{command}" > /dev/null$) {
-        return true
+    $[ -x "\$(command -v {command})" ]$ failed {
+        return false
     }
-    return false
+    return true
 }
 
 pub fun create_symbolic_link(origin: Text, destination: Text): Bool {

src/tests/stdlib.rs

@@ -448,11 +448,15 @@ fn is_command() {
         import { is_command } from \"std\"
         main {
             if is_command(\"cat\") {
-                echo \"exist\"
+                echo \"exist-cat\"
+            }
+
+            if is_command(\"this_is_not_command_amber\") {
+                echo \"exist-error\"
             }
         }
     ";
-    test_amber!(code, "exist")
+    test_amber!(code, "exist-cat")
 }
 
 #[test]