build: fix mksnapshot path in mksnapshot_args for flattened macOS zips (#53363)

Since the macOS release snapshot tools moved into a secondary toolchain
(#52884), gn desc emits the mksnapshot executable path as
./clang_<arch>_snapshot/mksnapshot, but electron_mksnapshot_zip flattens
mksnapshot.zip so the binary lives at the zip root. The Fixup Mksnapshot
step never rewrote that first argument, so every darwin mksnapshot.zip
published since then ships an mksnapshot_args pointing at a path that
does not exist in the zip and @electron/mksnapshot fails with ENOENT.

Rewrite the leading ./clang_*_snapshot/ prefix to ./ so the args file
matches the flattened layout. Linux is unaffected: its cross-arch
toolchain dir (clang_x64_v8_arm64) does not match the pattern and its
zips are not flattened.

Co-authored-by: Claude <noreply@anthropic.com>

.github/actions/build-electron/action.yml

@@ -261,6 +261,12 @@ runs:
         sed $SEDOPTION '/--reorder-builtins/d' out/Default/mksnapshot_args
         sed $SEDOPTION '/--warn-about-builtin-profile-data/d' out/Default/mksnapshot_args
         sed $SEDOPTION '/--abort-on-bad-builtin-profile-data/d' out/Default/mksnapshot_args
+        # When mksnapshot is built in a secondary toolchain (macOS release
+        # builds, see //electron/build/toolchain/mac), gn desc emits its path
+        # as ./clang_<arch>_snapshot/mksnapshot, but electron_mksnapshot_zip
+        # flattens the zip so the binary lives at the root. Point the args at
+        # the flattened location so @electron/mksnapshot can spawn it.
+        sed $SEDOPTION 's|^\./clang_[a-z0-9_]*_snapshot/mksnapshot|./mksnapshot|' out/Default/mksnapshot_args
 
         if [ "$TARGET_PLATFORM" = "win" ]; then
           cd out/Default