Prefer the `alt` build when possible for commits on or after b7ccb0a5a7ee5bf51dbb63490c0d7166cfb993e2

[kennytm]

In rust-lang/rust#45810 (merged as rust-lang/rust@b7ccb0a, with commit date 2017-11-06T17:57:51Z), the assertions are transferred to the "alt" builds. When doing bisection, it is preferable to have assertions enabled. Thus we should use the alt builds when possible, as suggested in rust-lang/rust#46449 (comment).