fix(ci): fetch minisign static binary (not in bullseye apt); bump alpha.7

alpha.6 signing failed: 'E: Unable to locate package minisign' —
minisign isn't packaged for node:20-bullseye. Download the official
static linux binary instead. Forward to alpha.7 (alpha.6 published
nothing).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

corrosion-host-agent/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "corrosion-host-agent"
-version = "2.0.0-alpha.6"
+version = "2.0.0-alpha.7"
 edition = "2021"
 description = "Corrosion Host Agent — multi-game ops runtime for self-hosted game servers"
 license = "UNLICENSED"