Update sccache binaries on CI

Currently CI builds can fail spuriously during the LLVM build (#39003). I
believe this is due to sccache, and I believe that in turn was due to the fact
that the sccache server used to just be a raw mio server. Historically raw mio
servers are quite complicated to get right, but this is why we built Tokio! The
sccache server has been migrated to Tokio which I suspect would fix any latent
issues.

I have no confirmation of this (never been able to reproduce the deadlock
locally), but my hunch is that updating sccache to the master branch will fix
the timeouts during the LLVM build.

The binaries previously came from Gecko's infrastructure, but I've built new
ones by hand for Win/Mac/Linux and uploaded them to our CI bucket.

appveyor.yml

@@ -115,11 +115,9 @@ install:
   - set PATH=C:\Python27;%PATH%
 
   # Download and install sccache
-  - appveyor DownloadFile https://api.pub.build.mozilla.org/tooltool/sha512/%SCCACHE_DIGEST%
-  - mv %SCCACHE_DIGEST% sccache.tar.bz2
-  - 7z x -y sccache.tar.bz2 > nul
-  - 7z x -y sccache.tar > nul
-  - set PATH=%PATH%;%CD%\sccache2
+  - appveyor DownloadFile https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-02-24-sccache-x86_64-pc-windows-msvc
+  - mv 2017-02-24-sccache-x86_64-pc-windows-msvc sccache
+  - set PATH=%PATH%;%CD%
 
   # Install InnoSetup to get `iscc` used to produce installers
   - choco install -y InnoSetup