Darwin has no syscall.FcntlInt, so main did not build on a Mac

The descriptor-passing change reached for syscall.FcntlInt, which exists in
x/sys/unix and not in Darwin's syscall package, so every build on a Mac has
failed since it landed. The last tag builds; main does not, which is the shape
that gets noticed late.

x/sys/unix is already required, so this adds nothing but the import.

Co-authored-by: Hanzo Dev <[email protected]>

files_cloexec_darwin.go

@@ -5,7 +5,8 @@ package zip
 import (
 	"os"
 	"runtime"
-	"syscall"
+
+	"golang.org/x/sys/unix"
 )
 
 // recvFlags carries no MSG_CMSG_CLOEXEC because Darwin has none. The flag is
@@ -15,8 +16,13 @@ import (
 const recvFlags = 0
 
 // setCloexec marks a received descriptor close-on-exec.
+//
+// Through x/sys/unix rather than syscall: FcntlInt is not in Darwin's syscall
+// package, so the file this replaces did not compile on a Mac at all — main has
+// been unbuildable there since the descriptor-passing change landed, while the
+// last tag builds. unix is already a dependency, so this adds nothing.
 func setCloexec(fd int) {
-	_, _ = syscall.FcntlInt(uintptr(fd), syscall.F_SETFD, syscall.FD_CLOEXEC)
+	_, _ = unix.FcntlInt(uintptr(fd), unix.F_SETFD, unix.FD_CLOEXEC)
 }
 
 // runtimeKeepFiles holds the files alive until the syscall has copied them.