freshBootstrapTools: rename from stdenvBootstrapTools

`stdenvBootstrapTools` was defined in both `nixpkgs` and `releases`
jobsets. This makes hydra view a bit confusing:

    $ git grep -F 'stdenvBootstrapTools =' | cat
    pkgs/top-level/all-packages.nix:  stdenvBootstrapTools = if stdenv.hostPlatform.isDarwin then
    pkgs/top-level/release.nix:      stdenvBootstrapTools = with lib;

The change renames jobset to be distinct. At least it improves grep
experience.

pkgs/top-level/all-packages.nix

@@ -2186,15 +2186,18 @@ with pkgs;
 
   brewtarget = libsForQt514.callPackage ../applications/misc/brewtarget { } ;
 
-  stdenvBootstrapTools = if stdenv.hostPlatform.isDarwin then
+  # Derivation's result is not used by nixpkgs. Useful for validation for
+  # regressions of bootstrapTools on hydra and on ofborg. Example:
+  #     pkgsCross.aarch64-multiplatform.freshBootstrapTools.build
+  freshBootstrapTools = if stdenv.hostPlatform.isDarwin then
     callPackage ../stdenv/darwin/make-bootstrap-tools.nix {
       localSystem = stdenv.buildPlatform;
       crossSystem =
         if stdenv.buildPlatform == stdenv.hostPlatform then null else stdenv.hostPlatform;
     }
   else if stdenv.hostPlatform.isLinux then
     callPackage ../stdenv/linux/make-bootstrap-tools.nix {}
-  else throw "stdenvBootstrapTools: unknown hostPlatform ${stdenv.hostPlatform.config}";
+  else throw "freshBootstrapTools: unknown hostPlatform ${stdenv.hostPlatform.config}";
 
   boxes = callPackage ../tools/text/boxes { };