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>

packages/bun-release/scripts/upload-npm.ts

@@ -73,9 +73,11 @@ async function buildRootModule(dryRun?: boolean) {
   });
   // Create placeholder scripts that print an error message if postinstall hasn't run.
   // On Unix, these are executed as shell scripts despite the .exe extension.
-  // On Windows, npm creates .cmd wrappers that would fail anyway if the binary isn't valid.
-  const placeholderScript = `#!/bin/sh
-echo "Error: Bun's postinstall script was not run." >&2
+  // Do NOT add a shebang (#!/bin/sh) here — npm's cmd-shim reads shebangs to generate
+  // .ps1/.cmd wrappers BEFORE postinstall runs, and bakes the interpreter path in.
+  // A #!/bin/sh shebang breaks Windows because the wrappers reference /bin/sh which
+  // doesn't exist, even after postinstall replaces the placeholder with the real binary.
+  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