Make getting the up-time more portable across unix-like systems

[qiu-x]

This fixes #324

[qiu-x]

@liamg

[qiu-x]

From https://pkg.go.dev/cmd/go#hdr-Build_constraints:

Go versions 1.16 and earlier used a different syntax for build constraints, with a "// +build" prefix. The gofmt command will add an equivalent //go:build constraint when encountering the older syntax.

I suggest updating the Go version used in the checks. For now I will change my fix to use the old syntax.

Edit: Never mind, that would be a pretty bad solution since the new Go version does not support the old syntax, so the build would fail on new versions. So the best way to fix the errors in the checks is updating Go