Reenable vault

`vault` was listed in `"GHC 8.8-related build failures"`, but that is
much too restrictive, since the library actually builds without issue
on GHC 8.8. The only part that doesn't build correctly is its
Haddocks, which is due to a Haddock bug (reported upstream at
haskell/haddock#1091). I've listed `vault` in
`expected-haddock-failures` as a result.

build-constraints.yaml

@@ -6020,7 +6020,6 @@ packages:
 
         # Misc
         - cuda < 0
-        - vault < 0
         - vivid-osc < 0
         - flay < 0
         - barbies < 0
@@ -6965,6 +6964,9 @@ expected-haddock-failures:
     - bins
     - emd
     - hmatrix-backprop
+
+    # https://github.com/haskell/haddock/issues/1091
+    - vault
 # end of expected-haddock-failures
 
 # For packages with haddock issues