Fix Windows cross-build: tzdata chmod ordering, ngtcp2 examples

tzdata: stdenv's unpackPhase runs `chmod -R u+w "$sourceRoot"` BEFORE
postUnpack, so the previous fix (creating tzdata-src in postUnpack)
failed because chmod ran on a nonexistent directory. Fix: set
dontMakeSourcesWritable=true and run chmod manually in postUnpack.

ngtcp2: examples include POSIX-only headers (sys/socket.h, sys/un.h)
that don't exist on Windows (same class of issue as nghttp3). Disable
examples with ENABLE_EXAMPLES=OFF.

Author: angerman

flake.nix

@@ -61,24 +61,23 @@
          # macOS builders via Rosetta, where Determinate Nix's sandbox
          # behaves differently from Linux.
          mingw = (final: prev: prev.lib.optionalAttrs prev.stdenv.hostPlatform.isWindows {
-           # tzdata tarballs extract into '.' (sourceRoot="."). On macOS
-           # builders with Determinate Nix, the sandbox places a read-only
-           # builder.json in the build directory. stdenv's genericBuild
-           # unconditionally runs `chmod +x $sourceRoot` after unpackPhase
-           # (to ensure cd works), which fails on '.' because the sandbox
-           # prevents permission changes on the build dir. Fix: extract
-           # into a subdirectory so sourceRoot != '.'.
+           # tzdata tarballs extract into '.' (no subdirectory). stdenv's
+           # unpackPhase runs `chmod -R u+w "$sourceRoot"` BEFORE postUnpack,
+           # so setting sourceRoot="tzdata-src" with a postUnpack to create
+           # the directory fails (chmod runs on nonexistent dir). Fix: skip
+           # the automatic chmod and handle it manually in postUnpack.
            tzdata = prev.tzdata.overrideAttrs (_: {
              sourceRoot = "tzdata-src";
+             dontMakeSourcesWritable = true;
              postUnpack = ''
                mkdir -p tzdata-src
-               # Move everything except builder.json into subdirectory
                for f in *; do
                  case "$f" in
                    tzdata-src|builder.json|env-vars) ;;
                    *) mv "$f" tzdata-src/ ;;
                  esac
                done
+               chmod -R u+w tzdata-src
              '';
            });
 
@@ -106,6 +105,15 @@
                (prev.lib.cmakeBool "ENABLE_LIB_ONLY" true)
              ];
            });
+
+           # ngtcp2 examples include POSIX-only headers (sys/socket.h,
+           # sys/un.h) that don't exist on Windows. The library itself
+           # builds fine; only the example client/server programs fail.
+           ngtcp2 = prev.ngtcp2.overrideAttrs (old: {
+             cmakeFlags = (old.cmakeFlags or []) ++ [
+               (prev.lib.cmakeBool "ENABLE_EXAMPLES" false)
+             ];
+           });
          });
 
          musl = (final: prev: prev.lib.optionalAttrs prev.stdenv.hostPlatform.isMusl {