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