In reply to @alex:tunstall.xyz
This seems to be caused by the unregisterised backend.
I haven't tried an unregisterised native build, but disabling unregisterised "fixed" the build, at the cost of producing segfaulting binaries.
I tried adding -std=gnu17 and even -std=gnu99 to EXTRA_CC_ARGS but I'm still getting errors related to the -std=gnu23 default, so I'm not sure what I'm missing here.

$ nix-build -A haskell.packages.ghc912.haskell-language-server --dry-run 2>&1 | grep "will be"
these 472 paths will be fetched (354.11 MiB download, 4469.75 MiB unpacked):

$ nix-build -A haskell.packages.ghc912.linear --dry-run 2>&1 | head -n 10
these 2 derivations will be built:
  /nix/store/k43nhxkm1wssqw6dcypgq911bjskchfd-bytes-0.17.5.drv
  /nix/store/49f28h8fg1bbf8mdgf2wpnv84r2jhp1y-linear-1.23.3.drv
these 81 paths will be fetched (31.83 MiB download, 402.85 MiB unpacked):

seems pretty much the same in current master:

$ nix-build -A haskell.packages.ghc912.haskell-language-server --dry-run 2>&1 | grep "will be"
these 507 paths will be fetched (357.33 MiB download, 4501.69 MiB unpacked):

$ nix-build -A haskell.packages.ghc912.linear --dry-run 2>&1 | head -n 4
these 2 derivations will be built:
  /nix/store/57bmq5jy0sraqqava0jr0swivma0xd0q-bytes-0.17.4.drv
  /nix/store/pbghk5rsl6s077p32m655yvn8c8fc246-linear-1.23.2.drv
these 100 paths will be fetched (36.87 MiB download, 465.34 MiB unpacked):

If I try to override Cabal and Cabal-syntax in a package set, why do I end up with multiple versions of Cabal? In particular I get this serious warning:

Warning:
    This package indirectly depends on multiple versions of the same package. This is very likely to cause a compile failure.
      package hackage-security (hackage-security-0.6.2.4-7CLHB9zPIN0FxCfhihIq1r) requires Cabal-syntax-3.10.3.0
      package Cabal (Cabal-3.12.1.0-HdYCe711lcf3vEdSKCpUQK) requires Cabal-syntax-3.12.1.0-JlhyVgsDFbHIDMshThsYz

Is it because those packages are not overridable (boot packages or such)? Or could it be configure-time dependencies?