This fixes the build:

config.status: creating hydra-config.h
config.status: executing depfiles commands
config.status: executing libtool commands
config.status: executing executable-scripts commands
building
build flags: -j8 -l8 SHELL=/nix/store/c4wxsn4jays9j31y5z9f83nr2cp7l4pa-bash-4.4-p23/bin/bash
make  all-recursive
make[1]: Entering directory '/build/source'
Making all in src
make[2]: Entering directory '/build/source/src'
Making all in hydra-evaluator
make[3]: Entering directory '/build/source/src/hydra-evaluator'
g++ -DHAVE_CONFIG_H -I. -I../..    -std=c++17 -I/nix/store/2xhb4hlskn33pbyph36v4wxcan56dnrw-boehm-gc-8.0.4-dev/include -I/nix/store/5rjgsqjdm71cflfb68q7m771a1rqcsk3-nix-2.4pre20200719_a79b6dd-dev/include/nix -Wall -I ../libhydra -Wno-deprecated-declarations -g -O2 -std=c++17 -include nix/config.h -c -o hydra_evaluator-hydra-evaluator.o `test -f 'hydra-evaluator.cc' || echo './'`hydra-evaluator.cc
hydra-evaluator.cc:27:27: error: template argument 1 is invalid
   27 |     std::unique_ptr<Config> config;
      |                           ^
hydra-evaluator.cc:27:27: error: template argument 2 is invalid
hydra-evaluator.cc: In constructor 'Evaluator::Evaluator()':
hydra-evaluator.cc:61:56: error: base operand of '->' is not a pointer
   61 |         , maxEvals(std::max((size_t) 1, (size_t) config->getIntOption("max_concurrent_evals", 4)))
      |                                                        ^~
hydra-evaluator.cc:60:44: error: invalid user-defined conversion from 'std::_MakeUniq<Config>::__single_object' {aka 'std::unique_ptr<Config, std::default_delete<Config> >'} to 'int' [-fpermissive]
   60 |         : config(std::make_unique<::Config>())
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~^~
In file included from /nix/store/3krz9s8ni3bqy4hy35ycmq8assrrb4f6-gcc-9.3.0/include/c++/9.3.0/memory:80,
                 from /nix/store/d97d0wixvlprz59z57maqj1pmda55r3b-libpqxx-6.4.5/include/pqxx/binarystring.hxx:17,
                 from /nix/store/d97d0wixvlprz59z57maqj1pmda55r3b-libpqxx-6.4.5/include/pqxx/binarystring:4,
                 from /nix/store/d97d0wixvlprz59z57maqj1pmda55r3b-libpqxx-6.4.5/include/pqxx/pqxx:3,
                 from ../libhydra/db.hh:3,
                 from hydra-evaluator.cc:1:
/nix/store/3krz9s8ni3bqy4hy35ycmq8assrrb4f6-gcc-9.3.0/include/c++/9.3.0/bits/unique_ptr.h:374:16: note: candidate is: 'std::unique_ptr<_Tp, _Dp>::operator bool() const [with _Tp = Config; _Dp = std::default_delete<Config>]' <near match>
  374 |       explicit operator bool() const noexcept
      |                ^~~~~~~~
/nix/store/3krz9s8ni3bqy4hy35ycmq8assrrb4f6-gcc-9.3.0/include/c++/9.3.0/bits/unique_ptr.h:374:16: note:   return type 'bool' of explicit conversion function cannot be converted to 'int' with a qualification conversion
make[3]: *** [Makefile:440: hydra_evaluator-hydra-evaluator.o] Error 1
make[3]: Leaving directory '/build/source/src/hydra-evaluator'
make[2]: *** [Makefile:360: all-recursive] Error 1
make[2]: Leaving directory '/build/source/src'
make[1]: *** [Makefile:414: all-recursive] Error 1
make[1]: Leaving directory '/build/source'
make: *** [Makefile:344: all] Error 2
builder for '/nix/store/g967cc3j6rc3nnpx2s4klpr03ig9zzyp-hydra-2020-06-23.drv' failed with exit code 2

hydra-unstable: 2020-06-23 -> 2020-07-28