revert: remove rust-toolchain

We use nix-shell to define the default version of rust (stable). If
people wish to "bring their own" they can use `--arg norust true`
or use `.envrc`.

Having a default rust-toolchain in the repo prevents this previous
behaviour from working correctly. And would require making changes
to the repo if one wants to override it.

I've updated the docs to reflect this.

doc/build.md

@@ -18,7 +18,7 @@ you won't need to worry about cross compiler toolchains, and all builds are repr
 
 ## Prerequisites
 
-Mayastor **only** builds on modern Linuxes. We'd adore contributions to add support for 
+Mayastor **only** builds on modern Linuxes. We'd adore contributions to add support for
 Windows, FreeBSD, OpenWRT, or other server platforms.
 
 If you do not have Linux system:
@@ -28,7 +28,7 @@ If you do not have Linux system:
 * **Mac:** We recommend you use [Docker for Mac][docker-install]
   and follow the Docker process described. Please let us know if you find a way to
   run it!
-* **FreeBSD:** We *think* this might actually work, SPDK is compatible! But, we haven't 
+* **FreeBSD:** We *think* this might actually work, SPDK is compatible! But, we haven't
   tried it yet.
 * **Others:** This is kind of a "Do-it-yourself" situation. Sorry, we can't be more help!
 
@@ -43,7 +43,7 @@ curl -L https://nixos.org/nix/install | sh
 > **Can't install Nix?**
 >
 > That's totally fine. You can use [`docker`][docker-install] just fine for one-off or occasional PRs!
-> 
+>
 > This flow will get you a pre-fetched `nix` store:
 > ```bash
 > docker run --name mayastor-nix-prefetch -it -v $(pwd):/scratch:rw --privileged --workdir /scratch nixos/nix nix-shell --run "exit 0"
@@ -75,16 +75,16 @@ First, setting the following:
 Then, updating the channel:
 
 ```bash
-$ sudo nix-channel --list 
+$ sudo nix-channel --list
 nixos https://nixos.org/channels/nixos-20.09
 $ sudo nix-channel --remove nixos
 $ sudo nix-channel --add https://nixos.org/channels/nixos-unstable nixos
 $ sudo nixos-rebuild switch --update
 ```
 
-> If you don't want, you can drop into a 
+> If you don't want, you can drop into a
 `nixUnstable` supporting shell with:
-> 
+>
 > ```bash
 > nix-shell -I nixpkgs=channel:nixpkgs-unstable -p nixUnstable --command "nix --experimental-features 'nix-command flakes' develop -f . mayastor"
 > ```
@@ -96,7 +96,7 @@ $ sudo nixos-rebuild switch --update
 
 
 You can use a tool like [`direnv`][direnv] to automate `nix shell` entry.
-
+If you are unable to use the Nix provided Rust for some reason, there are `norust` and `nospdk` arguments to Nix shell. `nix-shell --arg norust true`
 ## Iterative Builds
 
 Contributors often build Mayastor repeatedly during the development process.
@@ -120,6 +120,8 @@ cargo build --release
 **Want to run or hack on Mayastor?** *You need more configuration!* See
 [running][doc-running], then [testing][doc-testing].
 
+To ensure you are aware of this, we greet you with a nice cow.
+
 ## Artifacts
 
 There are a few ways to build Mayastor! If you're hacking on Mayastor, it's best to use