Bump golang.org/x/sys to fix building the project for Mac M1 machines

[uvegla]

This fixes building the project for Mac M1 machines. Example output for make build before the version bump of golang.org/x/sys:

mkdir bin
go build -o bin/kubeval .
# golang.org/x/sys/unix
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/syscall_darwin.1_13.go:25:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.1_13.go:27:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.1_13.go:40:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.go:28:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.go:43:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.go:59:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.go:75:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.go:90:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.go:105:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.go:121:3: //go:linkname must refer to declared function or variable
../../../../pkg/mod/golang.org/x/sys@v0.0.0-20200223170610-d5e6a3e2c0ae/unix/zsyscall_darwin_arm64.go:121:3: too many errors

See: golang/go#51091