eval-cache: fix parsing of contexts

[ConnorBaker]

Motivation

Fixing a small correctness bug -- the delimiter used for serialization and parsing should match.

Context

Storage of string contexts in the evaluation cache was introduced with 50f13b0. Currently it looks like:

nix/src/libexpr/eval-cache.cc

Lines 139 to 161 in b17034b

	AttrId setString(AttrKey key, std::string_view s, const Value::StringWithContext::Context * context = nullptr)
	{
	return doSQLite([&]() {
	auto state(_state->lock());
	if (context) {
	std::string ctx;
	bool first = true;
	for (auto * elem : *context) {
	if (!first)
	ctx.push_back(' ');
	ctx.append(elem->view());
	first = false;
	}
	state->insertAttributeWithContext.use()(key.first)(symbols[key.second])(AttrType::String) (s) (ctx)
	.exec();
	} else {
	state->insertAttribute.use()(key.first)(symbols[key.second])(AttrType::String) (s).exec();
	}
	return state->db.getLastInsertedRowId();
	});
	}

However, the delimiter used for parsing doesn't match:

nix/src/libexpr/eval-cache.cc

Lines 265 to 271 in b17034b

	case AttrType::String: {
	NixStringContext context;
	if (!queryAttribute.isNull(3))
	for (auto & s : tokenizeString<std::vector<std::string>>(queryAttribute.getStr(3), ";"))
	context.insert(NixStringContextElem::parse(s));
	return {{rowId, string_t{queryAttribute.getStr(2), context}}};
	}

Since this change doesn't modify the values written to the cache there's no need to bump the cache version.

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[Ericson2314]

I think this is correct, but it would be good for someone more familiar with the eval cache to double check