test suite: DedupUsingConfigFromSimple is flaky

[ulysses4ever]

We have CI hiccups lately because of DedupUsingConfigFromSimple. E.g https://github.com/haskell/cabal/actions/runs/15456775661/job/43510406325?pr=10854

I propose to mark it flaky ASAP and then investigate. /cc @philderbeast

[philderbeast]

OK by me.

[ulysses4ever]

Oh, it's another instance of the Stackage problem, see #10927...

2025-06-05T02:23:35.1768919Z -----BEGIN CABAL OUTPUT-----
2025-06-05T02:23:35.1769322Z Error: [Cabal-7111]
2025-06-05T02:23:35.1769668Z -----END CABAL OUTPUT-----
2025-06-05T02:23:35.1770059Z -----BEGIN CABAL OUTPUT-----
2025-06-05T02:23:35.1770719Z failed to download https://www.stackage.org/lts-21.25/cabal.config : HTTP code 502
2025-06-05T02:23:35.1771404Z -----END CABAL OUTPUT-----
2025-06-05T02:23:35.1771807Z CallStack (from HasCallStack):
2025-06-05T02:23:35.1772867Z   dieWithException, called at src/Distribution/Client/HttpUtils.hs:232:11 in cabal-install-3.15.0.0-inplace:Distribution.Client.HttpUtils
2025-06-05T02:23:35.1773764Z 
2025-06-05T02:23:35.1773771Z 
2025-06-05T02:23:35.1773785Z 
2025-06-05T02:23:35.1774325Z *** unexpected failure for PackageTests/ProjectImport/DedupUsingConfigFromSimple/cabal.test.hs
2025-06-05T02:23:35.1775140Z