Use -threaded for iserv-proxy

[hamishmack]

Fixes #2361

@luite tracked the failure down:

It's an async IO call that fails, async and fdwait have some limitations on Windows. the threaded runtime doesn't use them

[thomasjm]

This actually broke some aarch64 cross-compilation for me. For example, using GHC 9.12.2 and trying to build the aarch64-multiplatform version of the lsp-types package (from an x86_64-linux machine):

lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> [433 of 452] Compiling Language.LSP.Protocol.Internal.Method ( generated/Language/LSP/Protocol/Internal/Method.hs, dist/build/Language/LSP/Protocol/Internal/Method.o )
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> ---> Starting iserv-proxy-interpreter on port 5914
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> ---| iserv-proxy-interpreter should have started on 5914
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> [434 of 452] Compiling Language.LSP.Protocol.Message.Method ( src/Language/LSP/Protocol/Message/Method.hs, dist/build/Language/LSP/Protocol/Message/Method.o )
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> Listening on port 5914
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> qemu: uncaught target signal 4 (Illegal instruction) - core dumped
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> iserv-proxy: Uncaught exception ghc-internal:GHC.Internal.IO.Exception.IOException:
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> {handle: <socket: 15>}: GHCi.Message.remoteCall: end of file
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> HasCallStack backtrace:
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu>   collectBacktraces, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:169:13 in ghc-internal:GHC.Internal.Exception
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu>   toExceptionWithBacktrace, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:89:42 in ghc-internal:GHC.Internal.Exception
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu>   throw, called at libraries/ghci/GHCi/Message.hs:673:16 in ghci-9.12.2-inplace:GHCi.Message
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> /nix/store/vv44dyf6yar3kyshnpmf2xz71mz44kgv-iserv-wrapper/bin/iserv-wrapper: line 13:  1412 Illegal instruction     /nix/store/j2crk2gim81vrbhjmxqz5hp43km15hn4-qemu-9.1.3/bin/qemu-aarch64 /nix/store/vqy85983mrhp7wjgcr2p
fzrvpxzw879c-iserv-proxy-exe-iserv-proxy-interpreter-aarch64-unknown-linux-gnu-9.3/bin/iserv-proxy-interpreter tmp $PORT $ISERV_ARGS
lsp-types-lib-lsp-types-aarch64-unknown-linux-gnu> <no location info>: error: External interpreter terminated (1)