Fix GCC 14 fixincludes failure + postgresql cache breaker v2

GCC 14's fixincludes step bakes /usr/include into the Makefile via
@BUILD_SYSTEM_HEADER_DIR@ configure-time substitution. On NixOS-based
builders (Determinate Nix Linux VM), /usr/include doesn't exist,
failing all x86_64-linux GCC builds and cascading to 32 Windows
cross-builds. --disable-fixincludes is safe for Nix (store-managed
headers) and already used by nixpkgs' standalone libgcc build.

Also bumps postgresql musl cache breaker to v2 to force new drv hashes
after deploying sync-before-registering to darwin builders.

Author: angerman

flake.nix

@@ -49,6 +49,21 @@
            happy = final.haskell.lib.compose.overrideCabal (drv: {
              disallowGhcReference = false;
            }) prev.happy;
+
+           # 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.
+           # --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.
+           gcc14 = prev.wrapCC (
+             prev.gcc14.cc.overrideAttrs (old: {
+               configureFlags = (old.configureFlags or []) ++ [ "--disable-fixincludes" ];
+             })
+           );
          });
 
          cddl-tools = (final: prev: {
@@ -172,9 +187,9 @@
              # Force new drv hash to bypass hydra queue runner's persistent
              # failure cache. Previous builds succeeded but outputs were not
              # registered in the Nix DB (Determinate Nix orphaned store path
-             # bug). Orphaned paths have been cleaned on all builders.
+             # bug). Orphaned paths cleaned and sync-before-registering deployed.
              preBuild = (old.preBuild or "") + ''
-               # cache breaker: orphaned store paths cleaned 2026-02-15
+               # cache breaker v2: sync-before-registering deployed 2026-02-17
                true
              '';
            });