stdenv: linux: inherit autoconf269 at stage2/stage3

autoconf269 was being rebuilt at stages 2 and 3. It is used only as a
native build input (to regenerate `configure` scripts in autotools-using
packages); its outputs never appear in the final stdenv's runtime closure.

Inheriting it from prevStage at stage2 and stage3 collapses 2 -> 1 build.

Build-only dependency, safe with respect to the `disallowedRequisites`
check on the final stdenv-linux.

pkgs/stdenv/linux/default.nix

@@ -459,6 +459,7 @@ in
           texinfo
           which
           nukeReferences
+          autoconf269
           ;
 
         # Avoids infinite recursion, as this is in the build-time dependencies of libc.
@@ -568,6 +569,7 @@ in
             libunistring
             libxcrypt
             nukeReferences
+            autoconf269
             ;
           # We build a special copy of libgmp which doesn't use libstdc++, because
           # xgcc++'s libstdc++ references the bootstrap-files (which is what