Get rid of virtual Goal::init()

Now, each class provides the initial coroutine by value. This avoids
some sketchy virtual function stuff, and will also be further put to
good use in the next commit.

src/libstore/build/derivation-goal.cc

@@ -25,7 +25,7 @@ namespace nix {
 
 DerivationGoal::DerivationGoal(const StorePath & drvPath,
     const OutputsSpec & wantedOutputs, Worker & worker, BuildMode buildMode)
-    : Goal(worker)
+    : Goal(worker, init())
     , useDerivation(true)
     , drvPath(drvPath)
     , wantedOutputs(wantedOutputs)
@@ -43,7 +43,7 @@ DerivationGoal::DerivationGoal(const StorePath & drvPath,
 
 DerivationGoal::DerivationGoal(const StorePath & drvPath, const BasicDerivation & drv,
     const OutputsSpec & wantedOutputs, Worker & worker, BuildMode buildMode)
-    : Goal(worker)
+    : Goal(worker, init())
     , useDerivation(false)
     , drvPath(drvPath)
     , wantedOutputs(wantedOutputs)

src/libstore/build/drv-output-substitution-goal.cc

@@ -12,7 +12,7 @@ DrvOutputSubstitutionGoal::DrvOutputSubstitutionGoal(
     Worker & worker,
     RepairFlag repair,
     std::optional<ContentAddress> ca)
-    : Goal(worker)
+    : Goal(worker, init())
     , id(id)
 {
     name = fmt("substitution of '%s'", id.to_string());

src/libstore/build/substitution-goal.cc

@@ -9,7 +9,7 @@
 namespace nix {
 
 PathSubstitutionGoal::PathSubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair, std::optional<ContentAddress> ca)
-    : Goal(worker)
+    : Goal(worker, init())
     , storePath(storePath)
     , repair(repair)
     , ca(ca)