CI: system-test fails to configure test suite

Recently, system-test has been failing on nightly head.hackage jobs (example) with:

cabal: Cannot test the test suite 'system-test' because the solver did not
find a plan that included the test suites for ghc-debugger-0.2.0.0. It is
probably worth trying again with test suites explicitly enabled in the
configuration in the cabal.project{.local} file. This will ask the solver to
find a plan with the test suites available. It will either fail with an
explanation or find a different plan that uses different versions of some
other packages. Use the '--dry-run' flag to see package versions and check
that you are happy with the choices.

<snip>

=> Testing system-test
=> Test of system-test failed (code 1)

<snip>

1 testsuites failed.
    system-test