Add linux_arm64 builds for Ebitengine and Bevy

Batocera and the ES-family distributions run on ARM as much as on
x86_64 — Raspberry Pi, Odroid, the retro handhelds — and a linux_x64
binary installs there but will not start. There was no Linux ARM asset
kind at all: KINDS had linux_x86 and linux_x64 and mac_arm64, but
nothing for 64-bit ARM Linux.

Registering the kind is deliberately separate from producing it: a
platform service only includes BuildLinuxArm64 once its pipeline builds
the artifact, otherwise /api/builds would report it missing for every
release. Hence Ebitengine and Bevy only. Godot needs a Linux arm64
export preset in each game repo first; LÖVE fuses an upstream AppImage
that ships x86_64 only; TIC-80's export command has no ARM target.

Ebitengine needs cgo on Linux, so binary_build gained a cross-compiler
argument — and unsets CC for native targets, otherwise a build after
the ARM one silently picks up the cross gcc.

Verified by cross-compiling both demos in the rebuilt images: each
produced a genuine AArch64 ELF (e_machine 183), not a silent fallback.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/api/Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: ../libs/ruby/warp_engine
   specs:
-    warp_engine (0.2.0)
+    warp_engine (0.3.0)
       apipie-rails
       blueprinter
       rails (>= 8.0)