testing: refer to $PWD rather than pwd

pwd will resolve symlinks in some cases. This test need to run
regardless of which host os the test is running on. Some hosts /bin is
a symlink ot /urs/bin

src/Command.zig

@@ -700,7 +700,11 @@ test "Command: custom working directory" {
         .cwd = "C:\\Windows\\System32",
     } else .{
         .path = "/bin/sh",
-        .args = &.{ "/bin/sh", "-c", "pwd" },
+        .args = &.{
+            "/bin/sh",
+            "-c",
+            "echo $PWD",
+        },
         .stdout = stdout,
         .cwd = "/bin",
     };