main.star: use gotip-ish as bootstrap for openbsd/riscv64

The openbsd/riscv64 port is newly added in Go 1.23, so it's not viable
to use Go 1.20.6 or Go 1.21.0 as its bootstrap version. Use the latest
tip development version available at this moment.

For golang/go#67105.

Change-Id: Ia72042065fac0babeab24afa44ee8937f9bc7f46
Reviewed-on: https://go-review.googlesource.com/c/build/+/588356
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Joel Sing <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dmitshur