Fix project dirs path

    ProjectDirs::from("miri", "miri", "miri")

would get you `miri\miri` on Windows and `miri.miri.miri` on macOS.

I'm assuming here that your intention was to have only a `miri` directory on every OS,
so we are using `ProjectDirs::from_path instead, which skips the logic that computes
the correct platform-specific paths altogether.

.appveyor.yml

@@ -36,7 +36,7 @@ build_script:
     - cargo install --all-features --force --path . --locked --offline
     # Get ourselves a MIR-full libstd, and use it henceforth
     - cargo miri setup
-    - set MIRI_SYSROOT=%USERPROFILE%\AppData\Local\miri\miri\cache\HOST
+    - set MIRI_SYSROOT=%USERPROFILE%\AppData\Local\miri\cache\HOST
 
 test_script:
     # Test miri

src/bin/cargo-miri.rs

@@ -248,7 +248,7 @@ fn setup(ask_user: bool) {
     }
 
     // Next, we need our own libstd. We will do this work in whatever is a good cache dir for this platform.
-    let dirs = directories::ProjectDirs::from("miri", "miri", "miri").unwrap();
+    let dirs = directories::ProjectDirs::from_path("miri").unwrap();
     let dir = dirs.cache_dir();
     if !dir.exists() {
         fs::create_dir_all(&dir).unwrap();