Do not add home bin path to PATH if it's already there

This is to allow users to control the order via PATH if they so desire.

Tested by preparing two different `cargo-foo` scripts in `$HOME/.cargo/bin` and `$HOME/bin`:

```
> env PATH="/usr/bin/:$HOME/bin:$HOME/.cargo/bin" ./target/debug/cargo foo
Inside ~/bin/
> env PATH="$HOME/.cargo/bin:/usr/bin/:$HOME/bin" ./target/debug/cargo foo
Inside ~/.cargo/bin/
> env PATH="/usr/bin/:$HOME/bin" ./target/debug/cargo foo
Inside ~/.cargo/bin/
```

and trailing slash:

```
> env PATH="$HOME/.cargo/bin/:/usr/bin/:$HOME/bin" ./target/debug/cargo foo
Inside ~/.cargo/bin/
> env PATH="/usr/bin/:$HOME/bin:$HOME/.cargo/bin/" ./target/debug/cargo foo
Inside ~/bin/
```

Fix rust-lang#11020

src/bin/cargo/main.rs

@@ -220,10 +220,30 @@ fn is_executable<P: AsRef<Path>>(path: P) -> bool {
 }
 
 fn search_directories(config: &Config) -> Vec<PathBuf> {
-    let mut dirs = vec![config.home().clone().into_path_unlocked().join("bin")];
-    if let Some(val) = env::var_os("PATH") {
-        dirs.extend(env::split_paths(&val));
-    }
+    let path_dirs = if let Some(val) = env::var_os("PATH") {
+        env::split_paths(&val).collect()
+    } else {
+        vec![]
+    };
+
+    let home_bin = config.home().clone().into_path_unlocked().join("bin");
+
+    // If any of that PATH elements contains `home_bin`, do not
+    // add it again. This is so that the users can control priority
+    // of it using PATH, while preserving the historical
+    // behavior of preferring it over system global directories even
+    // when not in PATH at all.
+    // See https://github.com/rust-lang/cargo/issues/11020 for details.
+    //
+    // Note: `p == home_bin` will ignore trailing slash, but we don't
+    // `canonicalize` the paths.
+    let mut dirs = if path_dirs.iter().any(|p| p == &home_bin) {
+        vec![]
+    } else {
+        vec![home_bin]
+    };
+
+    dirs.extend(path_dirs);
     dirs
 }