Bump which to version 4 removing failure dependency (#55)

We are now completely free of failure dependencies!

src/tool/mod.rs

@@ -6,7 +6,6 @@ pub use chroot::chroot;
 pub use mount::mount;
 pub use qemu::qemu;
 
-use anyhow::anyhow;
 use std::path::PathBuf;
 use std::process::Command;
 use which::which;
@@ -18,16 +17,7 @@ pub struct Tool {
 
 impl Tool {
     pub fn find(name: &'static str) -> anyhow::Result<Self> {
-        // Note this conversion is only necessary until which releases their new version using
-        // thiserror instead of failure - then we can just use .with_context() on the thiserror
-        // Error
-        // Commit pending release:
-        // BLOCKED: https://github.com/harryfei/which-rs/commit/e6e839c4f6cdf8d3e33ec7eafdd50d34472740ea
-        let which = match which(name) {
-            Ok(x) => Ok(x),
-            Err(_) => Err(anyhow!("Could not find tool: {}", name)),
-        }?;
-        Ok(Self { exec: which })
+        Ok(Self { exec: which(name)? })
     }
 
     pub fn execute(&self) -> Command {