Prepare for CA derivation support with lower impact changes

This is just C++ changes without any Perl / Frontend / SQL Schema
changes.

The idea is that it should be possible to redeploy Hydra with these
chnages with (a) no schema migration and also (b) no regressions. We
should be able to much more safely deploy these to a staging server and
then production `hydra.nixos.org`.

Extracted from #875

Co-Authored-By: Théophane Hufschmitt <theophane.hufschmitt@tweag.io>
Co-Authored-By: Alexander Sosedkin <monk@unboiled.info>
Co-Authored-By: Andrea Ciceri <andrea.ciceri@autistici.org>
Co-Authored-By: Charlotte 🦝 Delenk Mlotte@chir.rs>
Co-Authored-By: Sandro Jäckel <sandro.jaeckel@gmail.com>

src/hydra-queue-runner/state.hh

@@ -520,7 +520,7 @@ private:
         const std::string & machine);
 
     int createSubstitutionStep(pqxx::work & txn, time_t startTime, time_t stopTime,
-        Build::ptr build, const nix::StorePath & drvPath, const std::string & outputName, const nix::StorePath & storePath);
+        Build::ptr build, const nix::StorePath & drvPath, const nix::Derivation drv, const std::string & outputName, const nix::StorePath & storePath);
 
     void updateBuild(pqxx::work & txn, Build::ptr build, BuildStatus status);
 
@@ -536,7 +536,7 @@ private:
     void processQueueChange(Connection & conn);
 
     BuildOutput getBuildOutputCached(Connection & conn, nix::ref<nix::Store> destStore,
-        const nix::Derivation & drv);
+        const nix::StorePath & drvPath);
 
     Step::ptr createStep(nix::ref<nix::Store> store,
         Connection & conn, Build::ptr build, const nix::StorePath & drvPath,