fix non-POSIX [[ ]]

[geekosaur]

Otherwise CI prints validate.sh: 332: [[: not found and the line does nothing (but validate.sh continues to run), unless the system shell is ksh / bash / zsh. This may explain https://github.com/haskell/cabal/pull/10114#issuecomment-2178163927.

Please read Github PR Conventions and then fill in one of these two templates.

---

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• [x] Patches conform to the coding conventions.
• [x] Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions). —only indirectly

[geekosaur]

Won't pass CI until #10172 lands.

[geekosaur]

@mergify backport 3.12

[mergify[bot]]

backport 3.12

✅ Backports have been created

• #10304 fix non-POSIX [[ ]] (backport #10174) has been created for branch 3.12 but encountered conflicts