top-level: Remove cycles: stdenv calls in top-level but not vice versa

This commit changes the dependencies of stdenv, and clean-up the stdenv
story by removing the `defaultStdenv` attribute as well as the `bootStdenv`
parameter.

Before, the final bootstrapping stage's stdenv was provided by
all-packages, which was iterating multiple times over the
top-level/default.nix expression, and non-final bootstrapping stages'
stdenvs were explicitly specified with the `bootStdenv` parameter.

Now, all stages' stdenvs are specified with the `stdenv` parameter.
For non-final bootstrapping stages, this is a small change---basically just
rename the parameter.
For the final stage, top-level/default.nix takes the chosen stdenv and
makes the final stage with it.

`allPackages` is used to make all bootstrapping stages, final and
non-final alike. It's basically the expression of `stage.nix` (along with a
few partially-applied default arguments)

Note, the make-bootstrap-tools scripts are temporarily broken

pkgs/stdenv/custom/default.nix

@@ -10,7 +10,7 @@ rec {
     inherit system platform crossSystem config;
     # It's OK to change the built-time dependencies
     allowCustomOverrides = true;
-    bootStdenv = vanillaStdenv;
+    stdenv = vanillaStdenv;
   };
 
   stdenvCustom = config.replaceStdenv { pkgs = buildPackages; };