Switch to Go 1.18

[oxtoacart]

No description provided.

[Crosse]

Hm, still failing. 😢 I think we'll need to do one of a few things:

1. don't run the race detector during tests here;
2. bring back the assembly versions of commitSleep() in netstack;
3. figure out if netstack/sleep/commit.go really needs the !race build directive and if not, remove that restriction.

I've tested all three options and they all produce succesful test runs with Go 1.18. I'd say option 2 is the way to go (barring a reason for not using the asm versions—there isn't one isn't mentioned in getlantern/netstack#7 but you may remember there being an issue with them), followed by option 3.

[oxtoacart]

@Crosse This fixes the test issue.