main.star: use Go 1.22.6 as bootstrap on tip (Go 1.24 to be)

For golang/go#64751.

Change-Id: I68b42b295f48a57f7016e8d70f068c7cb3548df8
Reviewed-on: https://go-review.googlesource.com/c/build/+/605978
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur