Adjust flags for to re-enable file-io (#7647)

Author: Bodigrim

build-constraints.yaml

@@ -6662,7 +6662,6 @@ packages:
         - fgl-arbitrary < 0 # tried fgl-arbitrary-0.2.0.6, but its *library* requires QuickCheck >=2.3 && < 2.15 and the snapshot contains QuickCheck-2.15.0.1
         - fib < 0 # tried fib-0.1.0.1, but its *library* requires the disabled package: base-noprelude
         - fields-and-cases < 0 # tried fields-and-cases-0.2.0.0, but its *library* requires base >=4.17.2.0 && < 4.20 and the snapshot contains base-4.20.0.0
-        - file-io < 0 # tried file-io-0.1.5, but its *library* requires filepath >=1.4.100.0 && < 1.5.0.0 and the snapshot contains filepath-1.5.2.0
         - filepath-bytestring < 0 # tried filepath-bytestring-1.5.2.0.0, but its *library* requires base >=4 && < 4.20 and the snapshot contains base-4.20.0.0
         - filtrable < 0 # tried filtrable-0.1.6.0, but its *library* requires containers >=0.5.11 && < 0.7 and the snapshot contains containers-0.7
         - first-class-patterns < 0 # tried first-class-patterns-0.3.2.5, but its *library* requires transformers >=0.1.0 && < 0.6 and the snapshot contains transformers-0.6.1.1
@@ -8092,6 +8091,9 @@ package-flags:
     # https://github.com/commercialhaskell/stackage/issues/7461
     hashable:
       arch-native: false
+
+    file-io:
+      os-string: true
 # end of package-flags
 
 # Special configure options for individual packages