feat: Add os and arch to the meta.* data sent (#200)

## Which problem is this PR solving?

We live in a multiarch world now! I'm currently thinking about migrating
a bunch of CI runners over from x86->arm64, and I can see a few uses for
having machine architecture in my buildevents data.

1. What parts of my CI runs are on an x86 vs arm executor? What's been
migrated, which spots are still to be done? (I expect this to be a
gradual migration as various dependencies - mostly docker images -
become available for arm.)

2. Performance improvements - can we compare run time of one arch versus
another?

Both of these will be easily answered by buildevents data if these
fields are present.

## Short description of the changes

Takes runtime.{GOOS,GOARCH}, and puts those values into `meta.os` and
`meta.arch`. I believe performance impact will be negligible - both of
these are constants set by the golang runtime on startup.

---
- To see the specific tasks where the Asana app for GitHub is being
used, see below:
  - https://app.asana.com/0/0/1205046216051392

common.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path"
 	"regexp"
+	"runtime"
 	"strconv"
 	"strings"
 	"time"
@@ -35,6 +36,9 @@ func createEvent(cfg *libhoney.Config, provider string, traceID string) *libhone
 	ev.AddField("trace.trace_id", traceID)
 	ev.AddField("meta.version", Version)
 
+	ev.AddField("meta.os", runtime.GOOS)
+	ev.AddField("meta.arch", runtime.GOARCH)
+
 	return ev
 }