kill a continue-on-error

It doesn't do what we really want, nor really does anything else
that isn't an unwieldy-at-best stack of conditions.

.github/workflows/validate.yml

@@ -261,6 +261,12 @@ jobs:
  echo End "$test"
  done
  exit $rc
+ # The above ensures all the tests get run, for a single platform+ghc.
+ # Trying to ensure they run for *all* combinations but still fail
+ # at the end seems to be extremely difficult at best. It's doable,
+ # but it requires a continuously growing stack of conditions and
+ # one possibly nightmarish final conditional. 'fail-fast' gets us
+ # partway there, at least, but is still imperfect.
 
  validate-old-ghcs:
  name: Validate old ghcs ${{ matrix.extra-ghc }}
@@ -320,8 +326,9 @@ jobs:
  - name: "Validate lib-suite-extras --extra-hc ghc-${{ matrix.extra-ghc }}"
  env:
  EXTRA_GHC: ghc-${{ matrix.extra-ghc }}
- continue-on-error: true
  run: sh validate.sh ${{ env.COMMON_FLAGS }} --lib-only -s lib-suite-extras --extra-hc "${{ env.EXTRA_GHC }}"
+ # See the comment above about running all tests but still failing if one
+ # of them does; it also applies here.
 
  build-alpine:
  name: Build statically linked using alpine