Build failure due to unstable feature use

[andreasabel]

$ cargo install agda-tree

   Compiling agda-tree v0.3.2
error[E0554]: `#![feature]` may not be used on the stable release channel
 --> /.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/agda-tree-0.3.2/src/lib.rs:1:1
  |
1 | #![feature(path_file_prefix)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0554`.

I am using version 1.86 of rust/cargo (latest on Homebrew).

It would be good if the package built with a standard install of rust.

[dannypsnl]

fixed at c636f61, that's actually not used now, so I remove it.