CI: Configure TZ to be GMT

[olleolleolle]

This makes one test pass.

This is a 🔨 code refactoring for developer experience.

Summary

By configuring the timezone in CI, we ensure that the tests are executed in a defined way.

I was able to make the test suite pass locally by making this change.

Context

See #9579 for details.

🔍 Notes

• jekyll/test/test_filters.rb:696 failed locally. Let's see if it fails here – with some kind of ordering issue? I ran the test on a Darwin Olles-Laptop.local 23.4.0 Darwin Kernel Version 23.4.0: Fri Mar 15 00:19:22 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T8112 arm64 with ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin23].

After focusing that test's output, by applying .sort to both actual and expected, I saw: the TZ

-"], ["date", "2008-02-02 00:00:00 +0000"], ["draft", false], ["excerpt", "<p>This should be published.</p>
+"], ["date", "2008-02-02 00:00:00 +0100"], ["draft", false], ["excerpt", "<p>This should be published.</p>

Failure:
TestFilters#test_: filters jsonify filter should convert drop to json.  [jekyll/test/test_filters.rb:696]
Minitest::Assertion: --- expected
+++ actual
@@ -1,27 +1,27 @@
 {
   "name": "2008-02-02-published.markdown",
+  "id": "/publish_test/2008/02/02/published",
   "path": "_posts/2008-02-02-published.markdown",
+  "date": "2008-02-02 00:00:00 +0100",
+  "relative_path": "_posts/2008-02-02-published.markdown",
   "previous": null,
-  "output": null,
+  "tags": [
+
+  ],
+  "url": "/publish_test/2008/02/02/published.html",
   "content": "This should be published.
 ",
-  "id": "/publish_test/2008/02/02/published",
-  "url": "/publish_test/2008/02/02/published.html",
-  "relative_path": "_posts/2008-02-02-published.markdown",
+  "title": "Publish",
+  "output": null,
   "collection": "posts",
   "excerpt": "<p>This should be published.</p>
 ",
-  "draft": false,
   "categories": [
     "publish_test"
   ],
+  "draft": false,
   "layout": "default",
-  "title": "Publish",
   "category": "publish_test",
-  "date": "2008-02-02 00:00:00 +0000",
   "slug": "published",
-  "ext": ".markdown",
-  "tags": [
-
-  ]
+  "ext": ".markdown"
 }