Cope with `FileLockingNotSupported` in Store logic

On some non-Windows archs (e.g. Solaris, as well as AIX depending on GHC version), `hLock` and `hTryLock` may (unconditionally) throw `FileLockingNotSupported`. This is a problem because the current nix-style store logic doesn't handle this exception thus on such platforms `cabal new-build` is currently unusable.

One simple way to simulate this issue on more mainstream archs is by modifying our `Distribution.Client.Compat.FileLock` module to have its exported `hLock` and `hTryLock` unconditionally throw `FileLockingNotSupported`.

Start with Distribution.Client.Compat.FileLock and the nix-style store logic used by cabal new-build; simulate the issue by making hLock and hTryLock throw FileLockingNotSupported. Trace where the exception reaches store operations, then verify that cabal new-build works on the simulated failure path and on supported platforms.

Written by the indexing model from the issue text.