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

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