Nix manual: access the doc output directly

we relied on an implementation detail of the Nix derivation (the `doc`
input attribute being passed to `builtins.derivation`, which is bound to
an environment variable in the derivation build process), and the
refactoring that happened betwen 2.25 and 2.26 made that fall apart.

nix/update-nix-releases.nix

@@ -47,12 +47,8 @@ writeShellApplication {
         fullVersion="$(<"$source"/.version)"
 
         # Only try versions recent enough to have the same structure, this filter out versions before 2.4
-        if ! drvPath=$(nix-instantiate "$source" --argstr system "x86_64-linux" -A default 2> /dev/null); then
-          echo >&2 "No default.nix supporting -A default"
-          break
-        fi
         # We need a doc output, this is always the case for >= 2.4
-        if ! docOutput=$(nix-store --query --binding doc "$drvPath"); then
+        if ! docOutput=$(nix-instantiate "$source" --argstr system "x86_64-linux" -A default.doc 2> /dev/null); then
           echo >&2 "No doc output"
           break
         fi