Fix is_absolute on WASI

WASI does not match `cfg(unix)`, but its paths are Unix-like (`/some/path`) and don't have Windows-like prefixes.

Without this change, `is_absolute` for paths like `/some/path` was returning `false`on a WASI target, which is obviously not true and undesirable.

library/std/src/path.rs

@@ -1838,7 +1838,7 @@ impl Path {
             // FIXME: Allow Redox prefixes
             self.has_root() || has_redox_scheme(self.as_u8_slice())
         } else {
-            self.has_root() && (cfg!(unix) || self.prefix().is_some())
+            self.has_root() && (cfg!(any(unix, target_os = "wasi")) || self.prefix().is_some())
         }
     }