appveyor: place cargo and rustup in the same cache

This avoids the problem that when one of the caches is restored and the
other isn't, Rust doesn't get reinstalled, but it also isn't usable,
crashing the CI.

.appveyor.yml

@@ -8,14 +8,15 @@ clone_depth: 1
 
 environment:
  APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
- CARGO_HOME: $(USERPROFILE)\.cargo
  CARGO_TARGET_DIR: $(APPVEYOR_BUILD_FOLDER)\target
  DENO_BUILD_MODE: release
  DENO_BUILD_PATH: $(APPVEYOR_BUILD_FOLDER)\out\release
  DENO_THIRD_PARTY_PATH: $(APPVEYOR_BUILD_FOLDER)\third_party
  MTIME_CACHE_DB: $(APPVEYOR_BUILD_FOLDER)\mtime_cache.xml
  RELEASE_ARTIFACT: deno_win_x64.zip
- RUSTUP_HOME: $(USERPROFILE)\.rustup
+ RUST_DIR: $(USERPROFILE)\rust
+ CARGO_HOME: $(RUST_DIR)\cargo
+ RUSTUP_HOME: $(RUST_DIR)\rustup
  RUST_BACKTRACE: 1
 
  # Appveyor uses 7zip to pack cache directories. We use these options:
@@ -225,8 +226,7 @@ for:
 
 cache:
  # Rust stuff.
- - $(CARGO_HOME)
- - $(RUSTUP_HOME)
+ - $(RUST_DIR)
  # Cache the third_party submodule to preserve binaries downloaded by setup.py,
  # and to make incremental builds work.
  - $(APPVEYOR_BUILD_FOLDER)\.git\modules\third_party