chore: support go1.18 and update dependencies (#708)

Here's the squash of the following patches that enable support
for go1.18 and update our dependencies.

This diff WILL need to be backported to the release/3.14 branch.

* chore: use go1.17.8

See https://github.com/ooni/probe/issues/2067

* chore: upgrade to probe-assets@v0.8.0

See https://github.com/ooni/probe/issues/2067.

* chore: update dependencies and enable go1.18

As mentioned in 7a0d17ea91,
the tree won't build with `go1.18` unless we say it does.

So, not only here we need to update dependencies but also we
need to explicitly say `go1.18` in the `go.mod`.

This work is part of https://github.com/ooni/probe/issues/2067.

* chore(coverage.yml): run with go1.18

This change will give us a bare minimum confidence that we're
going to build our tree using version 1.18 of golang.

See https://github.com/ooni/probe/issues/2067.

* chore: update user agent used for measuring

See https://github.com/ooni/probe/issues/2067

* chore: run `go generate ./...`

See https://github.com/ooni/probe/issues/2067

* fix(dialer_test.go): make test work with go1.17 and go1.18

1. the original test wanted the dial to fail, so ensure we're not
passing any domain name to exercise dialing not resolving;

2. match the end of the error rather than the whole error string.

Tested locally with both go1.17 and go1.18.

See https://github.com/ooni/probe-cli/pull/708#issuecomment-1096447186

internal/netxlite/dialer_test.go

@@ -62,8 +62,8 @@ func TestDialerSystem(t *testing.T) {
 			d := &dialerSystem{}
 			ctx, cancel := context.WithCancel(context.Background())
 			cancel() // immediately!
-			conn, err := d.DialContext(ctx, "tcp", "dns.google:443")
-			if err == nil || err.Error() != "dial tcp: operation was canceled" {
+			conn, err := d.DialContext(ctx, "tcp", "8.8.8.8:443")
+			if err == nil || !strings.HasSuffix(err.Error(), "operation was canceled") {
 				t.Fatal("unexpected err", err)
 			}
 			if conn != nil {