Bump sys module to fix darwin_arm64 build errors

[yankcrime]

This currently fails to build on Apple Silicon-based Macs with an error along the lines of the following:

/Users/nick/src/golang/pkg/mod/golang.org/x/sys@v0.0.0-20191022100944-742c48ecaeb7/unix/zsyscall_darwin_arm64.go:105:3: //go:linkname must refer to declared function or variable
/Users/nick/src/golang/pkg/mod/golang.org/x/sys@v0.0.0-20191022100944-742c48ecaeb7/unix/zsyscall_darwin_arm64.go:121:3: //go:linkname must refer to declared function or variable
/Users/nick/src/golang/pkg/mod/golang.org/x/sys@v0.0.0-20191022100944-742c48ecaeb7/unix/zsyscall_darwin_arm64.go:121:3: too many errors

Bumping the sys module fixes the issue and builds successfully.