os.execShellCmd won't compile on IOS
### Description

It can be reproduced when compiling this code using the IOS SDK.

```
import browsers

openDefaultBrowser("https://github.com/nim-lang/Nim/issues")
```
```
    nim c --noMain:on --app:staticlib -d:ios --os:ios
```

The issue I have here is that `openDefaultBrowser` will eventually call `system` imported from `stdlib.h`. This symbol has been deprecated on IOS and will fail with `Use posix_spawn APIs or NSTask instead. (On iOS, process spawning is unavailable.)`

https://github.com/nim-lang/Nim/blob/a18dcca744dff3d7dcdcf51737ae2a1b05728150/lib/pure/os.nim#L210-L211

### Nim Version

Nim Compiler Version 2.0.12 [MacOSX: arm64]
Compiled at 2025-02-26
Copyright (c) 2006-2023 by Andreas Rumpf

git hash: ce7c6f4f3365db2cc63bdd9d460c71ed937ee9e9

### Current Output

Build fails with:
```text
Use posix_spawn APIs or NSTask instead. (On iOS, process spawning is unavailable.)
```

### Expected Output

```text

Build succeeded.
The default browser is opened when the code runs.

```

### Known Workarounds

_No response_

### Additional Information

_No response_