Add option to disable sandbox

[ayakael]

Fixes #18

When building AKMS packages in chroot or LXC, sandboxing isn't always possible. This adds use_sandbox boolean and --[no]-sandbox option to AKMS to disable sandboxing.

[ayakael]

A simpler implementation is reverting 6a37404, which would look like this:

From d9dadb254cc2fede48942bdfb883a4bab3430160 Mon Sep 17 00:00:00 2001
From: Antoine Martin <dev@ayakael.net>
Date: Thu, 5 Jun 2025 14:38:07 -0400
Subject: [PATCH] Only run builds in bublewrap sandbox when using overlayfs

Reverts 6a3740411f7d7c515250261225483b67959b799b by only setting
`--sandbox` variable when `build_root` is set, which is set only when
overlayfs is used.

This fixes using `akms` in chroot environment, as bubblewrap breaks in
that context due to lack of namespace.

diff --git a/akms b/akms
index ffa92ad..77b04d7 100755
--- a/akms
+++ b/akms
@@ -593,8 +593,7 @@ build_module() {
 	chown -R "$BUILD_USER" "$builddir" || return 1
 
 	runas "$BUILD_USER" \
-		--sandbox "${build_root:-/}" \
-		--bind "$builddir" "$builddir" \
+		${build_root:+--sandbox "${build_root:-/}" --bind "$builddir" "$builddir"} \
 		srcdir="$srcdir" \
 		builddir="$builddir" \
 		kernel_ver="$kernel" \
-- 
2.49.0

This approach thus implicitely disables sandboxing when overlayfs is disabled. If this is a preferred approach, I can change the PR accordingly.

[ayakael]

I've tested with and without sandbox, and it works. Ready for review.