sys: BPF token discovery rejects Android

Describe the bug

Starting with v0.22.0, creating BPF maps or programs on Android fails during automatic BPF token discovery. The returned error contains:

bpf token: not supported on android

Android builds use the Linux kernel ABI and select Go's Linux-specific source files, but platform.IsLinux currently checks runtime.GOOS == "linux". runtime.GOOS is "android", so findToken() returns before the bpf() syscall is attempted.

I expected token discovery to run on Android and fall back to creating the BPF object without a token when BPF_TOKEN_CREATE is unavailable.

The same workload reaches the kernel and works on the same device with v0.21.0.

How to reproduce

1. Build an Android binary using github.com/cilium/ebpf v0.22.0.
2. On a rooted Android device with BPF support, create a map or load a collection.
3. Observe that creation fails with "bpf token: not supported on android" before the BPF object is passed to the kernel.

Version information

github.com/cilium/ebpf v0.22.0