survey: Override ICU to have static libs

[nh2]

TODO

• [ ] Potentially concerning: http://userguide.icu-project.org/packaging

  Users of your ICU must compile with -DU_STATIC_IMPLEMENTATION. Does that really mean that all downstream projects have to pass -DU_STATIC_IMPLEMENTATION to their C preprocessor?

• [ ] Override ALL icu derivations, not just icu64.

CC @bgamari @angerman

[nh2]

CI to track: https://buildkite.com/nh2/static-haskell-nix/builds/428#6679cc2c-139b-4f47-bcd8-0722fd0569e1

Looks like not much depends on icu64, only this rebuilds:

these derivations will be built:
  /nix/store/v2k2yavnwn4618mkc934432iiz6229j9-icu4c-64.2.drv
  /nix/store/jhf9wkcydy4xacbjyma0zzfbigjwy9gq-postgresql-11.6.drv
  /nix/store/rf4piwfwc5m2jnbn10vl6bicnaz9zb9n-postgresql-libpq-0.9.4.2.drv
  /nix/store/581h6891gwv2p4xd0sdryhkxyidbhidm-postgresql-simple-0.6.2.drv
  /nix/store/3vm78p9wf4ylfrv3h72h9w62g3jnygsh-pg-harness-server-0.6.2.drv
  /nix/store/8mixv2rrlhlpwx61yvpvqqzbwihi7nlh-postgresql-schema-0.1.14.drv
  /nix/store/hz6r28x475d6g1a8sj9yniklncfmlam9-postgresql-simple-migration-0.1.15.0.drv
  /nix/store/lxraw9v3bknwnpv9lk603hfb919x8giv-postgresql-orm-0.5.1.drv