Fix default gem detection for nested out-of-tree builds

The "# default:" marker records the source path relative to the build
directory, but default_gem? re-prefixed it with srcdir, resolving a
non-existent path and wrongly removing every default gemspec whenever
srcdir is deeper than one level. The sibling layout used on CI
(srcdir=../src) only canceled the double prefix by accident.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tool/outdate-bundled-gems.rb

@@ -101,7 +101,7 @@ class Removal
     (@defaults ||= {}).fetch(spec) do
       File.open(prefixed(spec)) do |f|
         if /^# default: (\S+) (\d+\.\d+)/ =~ f.gets("")
-          File.mtime(prefixed($1)) <= Time.at(Rational($2))
+          File.mtime($1) <= Time.at(Rational($2))
         else
           false
         end