Resync fixture lockfile with bumped Jekyll gemspec constraint

The gemspec bump in a71484f (`jekyll '~> 4.0'` → `'~> 4.4'`) left
spec/fixtures/test_site/Gemfile.lock pinning the old dependency
under the PATH gem block (`cheesy-gallery (1.1.1)` →
`jekyll (~> 4.0)`). ruby/setup-ruby@v1 runs `bundle install` with
frozen mode set, so the test-site CI matrix refused to install with
"The gemspecs for path gems changed, but the lockfile can't be
updated because frozen mode is set". Regenerate the lock so the
PATH gem's transitive constraint matches the gemspec; no resolved
gem versions move.

Author: claude

spec/fixtures/test_site/Gemfile.lock

@@ -2,7 +2,7 @@ PATH
   remote: ../../..
   specs:
     cheesy-gallery (1.1.1)
-      jekyll (~> 4.0)
+      jekyll (~> 4.4)
       rmagick (>= 4, < 6)
 
 GEM