appveyor: Use VS2017 for all our images

This was [recommended by AppVeyor][1] to see if it has any impact on our
build times, hopefully on the beneficial side of things! This shouldn't
affect our binary compatibility for generated compilers like it would
normally do for Linux.

[1]: https://help.appveyor.com/discussions/questions/29832-did-recent-changes-apply-to-possibly-slow-down-builds#comment_46484879

appveyor.yml

@@ -1,5 +1,8 @@
 environment:
-  SCCACHE_DIGEST: f808afabb4a4eb1d7112bcb3fa6be03b61e93412890c88e177c667eb37f46353d7ec294e559b16f9f4b5e894f2185fe7670a0df15fd064889ecbd80f0c34166c
+  # This is required for at least an AArch64 compiler in one image, and is
+  # otherwise recommended by AppVeyor currently for seeing if it has any
+  # affect on our job times.
+  APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017 Preview
 
   # By default schannel checks revocation of certificates unlike some other SSL
   # backends, but we've historically had problems on CI where a revocation
@@ -82,7 +85,6 @@ environment:
     DIST_REQUIRE_ALL_TOOLS: 1
     DEPLOY: 1
     CI_JOB_NAME: dist-x86_64-msvc
-    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017 Preview
   - RUST_CONFIGURE_ARGS: >
       --build=i686-pc-windows-msvc
       --target=i586-pc-windows-msvc