fix: use Process.platform for iOS detection instead of Swift.available

[nairpaa]

The script currently uses Swift.available to detect iOS, but this can fail if the Swift bridge crashes during initialization, leaving Swift as undefined:

Before (Frida 17.9.1):

[+] Arch: arm64
[+] Platform:  darwin
[ ] Locating Flutter library 1/5
Error: access violation accessing 0x267bdeb81
    at get flags (/frida/bridges/swift.js:1)
    at getModuleContext (/frida/bridges/swift.js:1)
    at getFullTypeName (/frida/bridges/swift.js:1)
TypeError: cannot read property 'available' of undefined
    at disableTLSValidation (disable-flutter-tls.js:123)

After:

[+] Arch: arm64
[+] Platform:  darwin
[ ] Locating Flutter library 1/5
[+] Flutter library located
[+] ssl_verify_peer_cert found at offset: 0x3b91a8 (0x3b91a8)
[+] ssl_verify_peer_cert has been patched

Change: Replace Swift.available with Process.platform === 'darwin' on line 123, which doesn't depend on the Swift runtime being available in the target process.

Test environment:

• iPhone X (iOS 16.7.14)
• Frida 17.9.1
• NVISO Flutter pinning demo app (eu.nviso.flutterPinning)