Reenable master build, unbreak CI

[Manishearth]

In #3371 we disabled the master build, however we currently need master for green CI.

This attempts to reenable the master build by installing rustup-toolchain-install-master with an older rustc, temporarily till rust-lang/rust#55376 is fixed

r? @phansch

[phansch]

Hmm, AppVeyor fails with:

set RUST_BACKTRACE=1
cargo build --features debugging
error: toolchain 'master' does not have the binary `cargo.exe`

I'm going to retrigger the build, maybe it was just spurious. I don't see how your change could have caused that.

[Manishearth]

Probably because the fallback works differently on windows or something. Perhaps we can install nightly and then install old-nightly on Windows

…

On Tue, Oct 30, 2018, 6:01 AM Philipp Hansch ***@***.***> wrote: Reopened #3383 <#3383>. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#3383 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABivSEIQK9d4J3lNjpEGxOw7HWOcBF6cks5up-smgaJpZM4YBKUZ> .

[matthiaskrgr]

If nrc's fix pr lands in the next rollup (ETA 1.5 hours from now) we can simply revert the commits and run from master again.

[flip1995]

Closed with #3384