fix(npm): remove shebang from placeholder scripts to fix npm i -g bun on Windows (#26517)

## Summary
- Removes the `#!/bin/sh` shebang from placeholder `bin/bun.exe` and
`bin/bunx.exe` scripts in the npm package
- Fixes `npm i -g bun` being completely broken on Windows since v1.3.7

## Problem
PR #26259 added a `#!/bin/sh` shebang to the placeholder scripts to show
a helpful error when postinstall hasn't run. However, npm's `cmd-shim`
reads shebangs to generate `.ps1`/`.cmd` wrappers **before** postinstall
runs, and bakes the interpreter path into them. On Windows, the wrappers
referenced `/bin/sh` which doesn't exist, causing:

```
& "/bin/sh$exe"  "$basedir/node_modules/bun/bin/bun.exe" $args
   ~~~~~~~~~~~~~
The term '/bin/sh.exe' is not recognized...
```

Even after postinstall successfully replaced the placeholder with the
real binary, the stale wrappers still tried to invoke `/bin/sh`.

## Fix
Remove the shebang. Without it, `cmd-shim` generates a direct invocation
wrapper that works after postinstall replaces the placeholder. On Unix,
bash/zsh still execute shebang-less files as shell scripts via ENOEXEC
fallback, so the helpful error message is preserved.

## Test plan
- [x] `bun bd test test/regression/issue/24329.test.ts` passes (2/2
tests)
- Manually verify `npm i -g bun` works on Windows

Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

test/regression/issue/24329.test.ts

@@ -13,8 +13,9 @@ test("bun npm placeholder script should exit with error if postinstall hasn't ru
 
   // This is the placeholder script content that gets written to bin/bun.exe
   // during npm package build (see packages/bun-release/scripts/upload-npm.ts)
-  const placeholderScript = `#!/bin/sh
-echo "Error: Bun's postinstall script was not run." >&2
+  // Note: no shebang — a #!/bin/sh shebang breaks Windows because npm's cmd-shim
+  // bakes the interpreter path into .ps1/.cmd wrappers before postinstall runs.
+  const placeholderScript = `echo "Error: Bun's postinstall script was not run." >&2
 echo "" >&2
 echo "This occurs when using --ignore-scripts during installation, or when using a" >&2
 echo "package manager like pnpm that does not run postinstall scripts by default." >&2
@@ -38,9 +39,11 @@ exit 1
   });
   expect(chmodExitCode).toBe(0);
 
-  // Run the placeholder script
+  // Run via sh explicitly — in real usage, bash/zsh automatically fall back to sh
+  // interpretation when execve returns ENOEXEC for a shebang-less executable file.
+  // Bun.spawn doesn't have that fallback, so we invoke sh directly here.
   await using proc = Bun.spawn({
-    cmd: ["./bun-placeholder", "--version"],
+    cmd: ["sh", "./bun-placeholder"],
     cwd: String(dir),
     env: bunEnv,
     stdout: "pipe",