make build-bin: on Linux, build with bun's pinned release build (closes #240)

nix's bun loads through a glibc in the nix store, and bun build --compile
copied that loader into bin/quak, so nix-collect-garbage could break the
cron backup. The build moves into script/build-bin. On Linux it downloads
bun 1.3.13's release archive for the machine from GitHub: x86-64
(baseline) or arm64, glibc or musl. It checks the archive's SHA-256
before running it, and compiles with that bun, so the binary loads
through the system loader. macOS still runs bun through nix-shell.

Model: opus-5-5

Makefile

@@ -26,10 +26,8 @@ check:
 build:
 	@script/build
 
-# Bundles the built dist/, so the binary reports the version script/build
-# stamped.
-build-bin: build
-	nix-shell -p bun --run "bun build dist/bin/quak.js --compile --outfile bin/quak"
+build-bin:
+	@script/build-bin
 
 install: build-bin
 	mkdir -p ~/bin