Scope gcc14 fixincludes override to x86_64-linux only

Using isLinux also changed aarch64-linux gcc14, which cascades through
the entire IFD chain (gcc14 → stdenv → glibc → linux-headers → GHC →
nix-tools → plan-nix). This forces linux-headers to be rebuilt, but
linux-headers fails on darwin builders' case-insensitive APFS (netfilter
headers like xt_TCPMSS.h vs xt_tcpmss.h collide). The fixincludes issue
only affects x86_64-linux GCC builds on the Determinate Nix Linux VM.

Author: angerman

flake.nix

@@ -49,19 +49,19 @@
            happy = final.haskell.lib.compose.overrideCabal (drv: {
              disallowGhcReference = false;
            }) prev.happy;
-         } // prev.lib.optionalAttrs prev.stdenv.hostPlatform.isLinux {
+         } // prev.lib.optionalAttrs (prev.stdenv.hostPlatform.system == "x86_64-linux") {
            # GCC 14's fixincludes step uses a configure-time substitution
            # @BUILD_SYSTEM_HEADER_DIR@ that bakes /usr/include into the
            # generated Makefile. On NixOS-based builders (Determinate Nix
-           # Linux VM), /usr/include doesn't exist, causing all x86_64-linux
-           # GCC builds to fail — which cascades to all Windows cross-builds
-           # that depend on the native GCC bootstrap compiler.
+           # Linux VM), /usr/include doesn't exist, causing x86_64-linux
+           # GCC builds to fail — cascading to all Windows cross-builds.
            # --disable-fixincludes skips the fixincludes step entirely, which
            # is safe for Nix where all headers are store-managed. Already
            # used by nixpkgs' standalone libgcc build for the same reason.
-           # Only applied on Linux — avoids unnecessary gcc14 rebuilds on
-           # Darwin (where gfortran fails against Apple SDK 14.4) and
-           # Windows (where the mingw cross-GCC doesn't have this issue).
+           # Scoped to x86_64-linux only: on Darwin, gfortran bootstrap fails
+           # (Apple SDK __FLT_EVAL_METHOD__); on aarch64-linux, changing gcc14
+           # cascades through the entire IFD chain, and linux-headers can't be
+           # rebuilt on darwin builders (case-insensitive APFS).
            gcc14 = prev.wrapCC (
              prev.gcc14.cc.overrideAttrs (old: {
                configureFlags = (old.configureFlags or []) ++ [ "--disable-fixincludes" ];