ci: fall back to the published interscript-maps gem when ../maps is absent

The rake workflow only checks out this repo, so the hard path
dependency made every bundle command fail with 'The path ../maps does
not exist' (exit 13). Mirrors the existing secryst-group pattern:
local sibling in dev, published gem in CI.

Author: ronaldtse

Gemfile

@@ -5,7 +5,11 @@ gemspec
 gem "rake", "~> 13.0"
 gem "rspec", "~> 3.13"
 
-gem "interscript-maps", path: "../maps"
+if File.exist?("../maps")
+  gem "interscript-maps", path: "../maps"
+else
+  gem "interscript-maps"
+end
 
 group :secryst do
   if File.exist?("../../secryst")