Build steam for aarch64 on Valve's native arm64 client

Arch ships steam for x86_64 only, so on AArch64 `omarchy-pkg-add steam`
found nothing. This aarch64 build installs the same Valve launcher files
(desktop entry, icons, steam-devices udev rules) but replaces the x86
bootstrap with Valve's native arm64 client, which then updates itself from
the stable steam_client_linuxarm64 channel.

The bootstrap is just the client binary, which links only glibc, taken
from the manifest's bins_linuxarm64_linuxarm64 zip and pinned to the sha2
the manifest publishes. /usr/bin/steam unpacks it on first launch, keeps
the ~/.steam links the client requires, and restarts the client when it
exits with 42 after updating. Valve's bin_steam.sh cannot do this: it only
knows the ubuntu12_32 bootstrap.

Beyond Arch's dependencies the client needs gtk2 and libibus for its UI,
and lsof, which it runs to find its own IPC ports.

pkgbuilds/steam/.omarchy/package.json

@@ -0,0 +1,3 @@
+{
+  "source": "local"
+}