Fix #117: Support bootstrap on read-only installations (e.g., Nix store)

[KaoruBB]

This PR fixes #117 by adding a writable environment fallback for the bootstrap process. It uses a hashed environment cache in DEPOT_PATH if the installation directory is read-only.

Fixes #117

[Copilot]

Pull request overview

This PR updates JuliaSnail’s bootstrap/environment handling to work when the installation directory is read-only (e.g., Nix store) by creating a writable cached environment under DEPOT_PATH and activating it during bootstrap.

Changes:

• Added pkg_env_for_bootstrap(dir) to redirect bootstrapping to a writable cached environment when dir is not writable.
• Updated @with_pkg_env to activate the selected environment, retry the action after instantiating/precompiling when bootstrap is required, and restore the previously active project afterward.
• Added bootstrap_required(err) to broaden bootstrap detection to include certain LoadError cases.

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[Copilot]

env_name is derived from hash(abspath(dir)), but hash is intentionally not stable across Julia sessions (seeded), so the cache directory name may change between runs. That defeats the purpose of a persistent bootstrap cache and can create a new environment on every startup. Consider using a deterministic digest of abspath(dir) (e.g., SHA-1/SHA-256 via the SHA stdlib) or another stable encoding instead.

[Copilot]

The cache location uses first(Base.DEPOT_PATH). If the first depot is not writable (or DEPOT_PATH is empty/misconfigured), mkpath(cache) / cp(...) will fail, which undermines the read-only-installation support this PR adds. Consider selecting the first writable depot in Base.DEPOT_PATH (or falling back to tempdir() with a clear error if none are writable).

Suggested change

	function pkg_env_for_bootstrap(dir)
	if iswritable(dir)
	return dir
	end
	env_name = "julia-snail-" * string(hash(abspath(dir)), base=16)
	cache = joinpath(first(Base.DEPOT_PATH), "environments", env_name)
	function first_writable_depot_or_tempdir()
	# Try to find a writable depot in Base.DEPOT_PATH
	for depot in Base.DEPOT_PATH
	if iswritable(depot)
	return depot
	end
	end
	# Fall back to a writable temporary directory
	tmp = tempdir()
	if iswritable(tmp)
	return tmp
	end
	error("JuliaSnail: no writable depot in Base.DEPOT_PATH and tempdir() is not writable; cannot create cache environment.")
	end
	function pkg_env_for_bootstrap(dir)
	if iswritable(dir)
	return dir
	end
	env_name = "julia-snail-" * string(hash(abspath(dir)), base=16)
	cache_root = first_writable_depot_or_tempdir()
	cache = joinpath(cache_root, "environments", env_name)