Don't pass duplicate /d flag in Windows cmd use-on-cd hook

.changeset/witty-drives-turn.md

@@ -0,0 +1,10 @@
+---
+"fnm": patch
+---
+
+fix `cd /d <drive>` failing in Windows Command Prompt when use-on-cd is enabled: the generated `cd.cmd` always prepended `/d`, so a user-supplied `/d` was passed twice and `cd` failed with "The syntax of the command is incorrect". Fixes #1556
+
+```sh-session
+> cd /d D:\
+The syntax of the command is incorrect.
+```

e2e/repros/issue-1556.test.ts

@@ -0,0 +1,22 @@
+import { writeFile, mkdir } from "node:fs/promises"
+import { join } from "node:path"
+import { script } from "../shellcode/script.js"
+import { WinCmd } from "../shellcode/shells.js"
+import testCwd from "../shellcode/test-cwd.js"
+import testNodeVersion from "../shellcode/test-node-version.js"
+import describe from "../describe.js"
+
+describe(WinCmd, () => {
+  test(`issue #1556: cd /d works with use-on-cd`, async () => {
+    const subdir = join(testCwd(), "subdir")
+    await mkdir(subdir, { recursive: true })
+    await writeFile(join(subdir, ".node-version"), "v12.22.12")
+    await script(WinCmd)
+      .then(WinCmd.env({ useOnCd: true }))
+      .then(WinCmd.call("fnm", ["install", "v8.11.3"]))
+      .then(WinCmd.call("fnm", ["install", "v12.22.12"]))
+      .then(WinCmd.call("cd", ["/d", subdir]))
+      .then(testNodeVersion(WinCmd, "v12.22.12"))
+      .execute(WinCmd)
+  })
+})

src/shell/windows_cmd/cd.cmd

@@ -1,5 +1,9 @@
 @echo off
-cd /d %*
+rem Add /d only when the user didn't pass it themselves, so `cd /d D:\` works (#1556).
+rem No if/else parentheses or setlocal here: %* may contain ")" (e.g. "C:\Program Files (x86)"),
+rem and endlocal would revert the directory change.
+if /i "%~1" == "/d" cd %*
+if /i not "%~1" == "/d" cd /d %*
 if "%FNM_VERSION_FILE_STRATEGY%" == "recursive" (
   fnm use --silent-if-unchanged
 ) else (