Fix broken OS detection

The rust-fuzz#141 workaround was actually fully disabled because the OS condition
was wrong. This fixes the workaround.

Author: pedrocr

src/bin/cargo-afl.rs

@@ -190,7 +190,7 @@ where
         common::afl_llvm_rt_dir().display()
     );
 
-    if cfg!(linux) {
+    if cfg!(target_os = "linux") {
         // work around https://github.com/rust-fuzz/afl.rs/issues/141 /
         // https://github.com/rust-lang/rust/issues/53945, can be removed once
         // those are fixed.