Use explicit version of cfg-if in msrv build

[KodrAus]

Just to try fix up our CI.

[KodrAus]

r? @sfackler

[sfackler]

It wasn't worth pinning cfg-if?

[KodrAus]

I didn't have a lot of success pinning cfg-if using the cargo update onn the 1.16 toolchain, but I didn't actually try setting a = bound in our own Cargo.toml. We didn't get a lot of mileage out of bumping our MSRV from 1.16 to 1.31 anyways, so I'll give that a go 👍

[KodrAus]

Ok, looks like that works out just fine!

[sfackler]

This will cause issues if log is used in a dependency graph with another crate with a non = dependency on cfg-if. Cargo will iirc just refuse to resolve versions. If we can't just cargo update -p cfg-if --precise 0.1.9 in the MSRV build, it may just be best to bump our MSRV to whatever cfg-if uses.

[KodrAus]

Ah right. I’d assumed we’d possibly just end up compiling two versions of cfg-if.

I’ll look again at why the cargo update path didn’t work out for me and fall back to bumping back to 1.31.

[sfackler]

rustfmt's failing, but r=me otherwise. Realistically, no-one's going to do the downgrade dance with cfg-if anyway...