Avoid -qg incompatible with 9.2 runtime

Index: hadrian/hadrian.cabal
--- hadrian/hadrian.cabal.orig
+++ hadrian/hadrian.cabal
@@ -180,7 +180,7 @@ executable hadrian
                        --        waiting for external processes
                        -- * -qg: Don't use parallel GC as the synchronization
                        --        time tends to eat any benefit.
-                       "-with-rtsopts=-I0 -qg"
+                       "-with-rtsopts=-I0"
                        -threaded
 
     if flag(selftest)
