testsuite: UntrimmedImport test often fails in CI lately

### Describe the bug
/
```
Error parsing project file https://www.stackage.org/nightly-2024-12-05/cabal.config:9:
- "" (line 2543, column 4):
unexpected end of input
expecting ".", ":", white space, "+", "-", opening paren, operator, "installed", "source", "test" or "bench"

...
*** unexpected failure for PackageTests/ProjectImport/UntrimmedImport/cabal.test.hs
```

Example run: https://github.com/haskell/cabal/actions/runs/14853023896/job/41700087129?pr=10946#step:17:3022

Apparently, this has to do with the recent migration of Stackage and changes in caching of these `cabal.config` files. @chreekat says it may be https://github.com/commercialhaskell/stackage-server/issues/347

### Notes

I don't think it's a good idea to have a test that downloads anything from the Internet. So, instead of "simply" marking it as flaky, we could instead fix it to avoid that.