remove CARGO_TARGET_DIR duplicate from Dockerfile

.devcontainer/Dockerfile

@@ -85,11 +85,6 @@ EOM
 # `cargo` and `rustup` effortlessly.
 ENV PATH="/rustup/bin:${PATH}"
 
-# Moreover, we do not want to interfere with Cargo's output
-# directory of the host (in case someone want to use both this container
-# image and the hst); hence, we use a different output directory.
-ENV CARGO_TARGET_DIR=dev-container/target
-
 # Moreover, we do not want to interfere with Cargo's output
 # directory of the host (in case someone want to use both this container
 # image and the hst); hence, we use a different output directory.