CI: re-enable io-testsuite

Closes #222

Author: sorki

cabal.project.local.ci

@@ -1,9 +1,6 @@
 tests: True
--- this is intentionally disabled
--- as it fails on ubuntu-20.04 and ubuntu-latest
--- with unshare: invalid argument
--- and it cannot run on darwin as it requires linux-namespaces
--- flags: +io-testsuite
+
+flags: +io-testsuite
 
 package hnix-store-core
   ghc-options: -Wunused-packages -Wall