stdenv: don't discard string context from ContentAddressed derivations

Without the change build for packages that use `disallowedReferences`
fails in `contentAddressedByDefault = true` mode:

    $ nix build -f. ruby_3_1 --arg config '{ contentAddressedByDefault = true; }'
    ...
    error: derivation contains an illegal reference specifier '/0j3hif3ni7zl5zhlzzr5q2q23z66136mnzp75pyiqp5c72q14im2'
    error: 1 dependencies of derivation '/nix/store/39ji7qp225pxvrm8cgvzmyjqsyis2n0h-ruby-3.1.2.drv' failed to build

Original intent of https://github.com/NixOS/nixpkgs/pull/211783 was to
avoid pulling in actual derivation for reference scanning purposes.

Unfortunately CA derivations's outputPath are placeholders until they
are instantiated.

Let's restore string context for CA derivations for now.

pkgs/stdenv/generic/make-derivation.nix

@@ -139,7 +139,7 @@ let
   # Turn a derivation into its outPath without a string context attached.
   # See the comment at the usage site.
   unsafeDerivationToUntrackedOutpath = drv:
-    if isDerivation drv
+    if isDerivation drv && (!drv.__contentAddressed or false)
     then builtins.unsafeDiscardStringContext drv.outPath
     else drv;