glbl_cfg: reload without mutating the original

[oliver-sanders]

Sibling: cylc/cylc-rose#336

• Use the new reload kwarg rather than calling the .load() method.
• Fixes glblcfg: platform not found #6244
• The .load() method mutates the existing config, due to the use of logging within (and outside of) this routine and the use of glbl_cfg in the logging, this created a race condition.
• The new reload kwarg creates a new config instance, then sets this as the default.

To assert that the test captures the issue, try applying this diff to test the old reload method:

diff --git a/tests/integration/test_config.py b/tests/integration/test_config.py
index 231fb12f8..eb56ed5cb 100644
--- a/tests/integration/test_config.py
+++ b/tests/integration/test_config.py
@@ -588,7 +588,9 @@ async def test_glbl_cfg(monkeypatch, tmp_path, caplog):
     # * but during the reload itself, the old config should persist
     #   see https://github.com/cylc/cylc-flow/issues/6244
     expect_platforms_during_reload({'localhost', 'foo'})
-    assert get_platforms(glbl_cfg(reload=True)) == {'localhost', 'foo', 'bar'}
+    cfg = glbl_cfg()
+    cfg.load()
+    assert get_platforms(cfg) == {'localhost', 'foo', 'bar'}
     assert 'ran expect_platforms_during_reload test' in caplog.messages
 
     # the cache should have been updated by the reload

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

👍🏼 I enjoy learning about stuff when I review and this is rather clever.

[wxtim]

• Happy with code
• Tried locally
• Test fails against 8.3.x (and not just because it has an unrecognized keyword arg)

[hjoliver]

LGTM!

(Two approvals, but @MetRonnie has an unresolved comment)