Todo

Sccache Stdenv Direction

Goal: move sccache to the same structural layer as ccacheStdenv, meaning between the Nix cc-wrapper and the real compiler, instead of wrapping the cc-wrapper script itself.
Reason: this should preserve normal compiler identity and wrapper semantics while still allowing cached compilations.
Constraint: prefer direct local-disk cache access for sandboxed builds and avoid depending on the external sccache client/server socket model.

Added a local patch to sccache so its GCC/Clang parser treats -cxx-isystem <dir> like -isystem <dir>.
Added sccache.links, modeled after nixpkgs ccache.links, to generate cc/c++/gcc/g++/clang/clang++ wrappers that call sccache with the real compiler path.
Added sccacheWrapper, modeled after nixpkgs ccacheWrapper.
Added sccacheStdenv, modeled after nixpkgs ccacheStdenv.
Reworked mkSccacheStdenv to delegate to sccacheWrapper instead of patching cc-wrapper scripts directly.
Kept direct-disk cache configuration through generated SCCACHE_CONF and SCCACHE_NO_DAEMON=1.
Adjusted the wrapper override helper so it can handle both normal cc-wrappers (cc = ...) and Intel SYCL wrappers (llvm = ...).
Forwarded Intel-expected outputs/attrs like .dev, .lib, and .rsrc through sccache.links.

The previous direct insertion into cc-wrapper final exec did not break hello-world compilation.
That previous attempt was not cache-effective because sccache interpreted wrapper-emitted -cxx-isystem arguments as extra input files and returned multiple input files.
The exact problematic arguments were confirmed with a direct sccache clang ... probe using the wrapper-emitted argv.
The new ccache-style sccacheWrapper / sccacheStdenv path evaluates successfully.
intel-sycl.stdenv.cc now also evaluates successfully with the ccache-style sccache.links replacement in place.
The custom kernel helper now normalizes config-only kernelPatches entries to { patch = null; ... } before calling buildLinux, which fixes devenv shell -- build failing during kernel evaluation.
The kernel build does invoke sccache during live builds, but relying on the in-sandbox wrapper to create /var/cache/sccache/nix-builds/kernel made host-side observation ambiguous.
The host NixOS config should create and expose /var/cache/sccache/nix-builds/{packages,kernel} explicitly so sandboxed builds and host-side inspection see the same cache paths.

electron-real-clang-base should stay store-pure: synthesize its output tree from symlinks only and never cp/rm against copied toolchain store content.
mkCmakeSccacheAttrs should not force raw compilers into CMake; the sccache.links wrappers should bypass CMake probe invocations while keeping the normal wrapper-selected compiler environment.
The probe bypass must inspect @responseFile arguments as well, because the cc-wrapper often hides CMakeFiles/CompilerId paths inside response files before the inner wrapped-sccache sees them.
For CMake packages, the practical fallback is to export SCCACHE_WRAPPED_COMPILER_PASSTHROUGH=1 for the whole configurePhase and unset it in postConfigure; caching matters in buildPhase, not in compiler-ID probes.
Apply the shared CMake helper to stalled subpackages too, not just top-level wrappers. oneapi-math-sycl-blas is one such case.
The same helper may also be needed on the outer package if the subpackage is fixed but the wrapper package then stalls at its own CMake configure.
Run a minimal hello-world derivation with the new sccacheStdenv and confirm repeated compiles produce cache hits.
If that works, switch current consumers to the ccache-style path and remove obsolete wrapper-patching logic.