VirtualTerminal: adjust write syscall handling on GNU

When building against the GNU Linux environment, `unistd` is not a valid
submodule, so update the `write` syscall spelling to `Glibc.write`
instead of the Darwin `unistd.write`.

Author: compnerd

Sources/VirtualTerminal/Platform/POSIXTerminal.swift

@@ -278,7 +278,11 @@ internal final actor POSIXTerminal: VTTerminal {
   /// - Screen clearing and scrolling commands
   /// - Other VT100/ANSI control sequences
   public func write(_ string: String) {
+#if GNU
+    _ = Glibc.write(self.hOut, string, string.utf8.count)
+#else
     _ = unistd.write(self.hOut, string, string.utf8.count)
+#endif
   }
 }