Disable sdist build in the docspec test

Author: harendra-kumar

.github/workflows/haskell.yml

@@ -235,6 +235,7 @@ jobs:
             ghc_version: 9.12.4
             # WARNING! cannot use # comments inside pack_options.
             pack_options: >-
+              DISABLE_SDIST_BUILD="y"
               ENABLE_DOCSPEC="y"
               DOCSPEC_URL=https://github.com/phadej/cabal-extras/releases/download/cabal-docspec-0.0.0.20250606/cabal-docspec-0.0.0.20250606-x86_64-linux.xz
               DOCSPEC_OPTIONS="--timeout 60"