lib.systems.extensions.sharedLibrary: do not throw

Because downstream code expects to use `==` on platform attrsets, we
are unfortunately not able to throw a useful error message when the
`sharedLibrary` attribute is accessed.

When users do a comparison like:

  stdenv.hostPlatform == pkgsStatic.stdenv.hostPlatform

... in a situation where `stdenv.hostPlatform.hasSharedLibraries`,
they expect this to return `false`.  Unfortunately Nix does a deep
equality comparison here, and ends up forcing the
`pkgsStatic.stdenv.hostPlatform.extensions.sharedLibrary` attribute,
which throws the error.

Rather than returning `null`, this commit instead simply omits the
`extensions.sharedLibrary` attribute.  This provides the user with a
more-useful error message: instead of waiting until the `null` is
used (and hoping that produces an error), the user will get an error
about the `extensions.sharedLibrary` attribute being missing, at the
position where it was referenced.

Big thanks to @trofi for his PR to add
`NIX_VALIDATE_EVAL_NONDETERMINISM` to Nix, which I am now using.  It
made tracking this down really easy!

Fixes #244045

lib/systems/default.nix

@@ -85,17 +85,18 @@ rec {
         # is why we use the more obscure "bfd" and not "binutils" for this
         # choice.
         else                                     "bfd";
-      extensions = rec {
+      extensions = lib.optionalAttrs final.hasSharedLibraries {
-        sharedLibrary = assert final.hasSharedLibraries;
+        sharedLibrary =
-          /**/ if final.isDarwin  then ".dylib"
+          if      final.isDarwin  then ".dylib"
           else if final.isWindows then ".dll"
           else                         ".so";
+      } // {
         staticLibrary =
           /**/ if final.isWindows then ".lib"
           else                         ".a";
         library =
-          /**/ if final.isStatic then staticLibrary
+          /**/ if final.isStatic then final.extensions.staticLibrary
-          else                        sharedLibrary;
+          else                        final.extensions.sharedLibrary;
         executable =
           /**/ if final.isWindows then ".exe"
           else                         "";