vendor: update syscalls dependency

(cherry picked from commit dcc045f03c)

vendor/golang.org/x/sys/unix/env_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
 
 // Unix environment variables.
 
@@ -25,3 +25,7 @@ func Clearenv() {
 func Environ() []string {
 	return syscall.Environ()
 }
+
+func Unsetenv(key string) error {
+	return syscall.Unsetenv(key)
+}