Add back +legacy-comparison to the validate project

[philderbeast]

A follow on from #12139 (depends-on: 12139), adding back +legacy-comparison to the validate project, something that was removed in #11773.

$ git diff HEAD~1 HEAD~0
diff --git a/cabal.validate.project b/cabal.validate.project
index d1d57e0fe..bf2552af2 100644
--- a/cabal.validate.project
+++ b/cabal.validate.project
@@ -13,7 +13,7 @@ program-options
 -- The +legacy-comparison flag is to set the default project file parser to compare
 -- the result of the legacy and parsec parser and fail if they are not equal.
 package cabal-install
-  flags: +git-rev
+  flags: +git-rev +legacy-comparison
 
 package Cabal
   flags: +git-rev

---

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