Pin cabal-experimental to pre-regression version

The cabal-experimental update from 7e50837 (Apr 2025) to bc52b09
(Aug 2025) introduced a change in pkgsUseSharedLibrary (commit
f47840db) that stopped checking the compiler's capabilities when
deciding whether to build shared libraries. This causes store
dependencies (e.g. os-string) to be installed without .dyn_hi files,
while project packages (e.g. tar) are still built with -dynamic-too,
resulting in missing dynamic interface file errors.

Pin back to the known-working version until the regression is
resolved upstream.

Author: angerman