nix: Add --print-build-logs flag

This causes 'nix' to print build log output to stderr rather than
showing the last log line in the progress bar. Log lines are prefixed
by the name of the derivation (minus the version string), e.g.

  binutils> make[1]: Leaving directory '/build/binutils-2.31.1'
  binutils-wrapper> unpacking sources
  binutils-wrapper> patching sources
  ...
  binutils-wrapper> Using dynamic linker: '/nix/store/kr51dlsj9v5cr4n8700jliyz8v5b2q7q-bootstrap-stage0-glibc/lib/ld-linux-x86-64.so.2'
  bootstrap-stage2-gcc-wrapper> unpacking sources
  ...
  linux-headers> unpacking sources
  linux-headers> unpacking source archive /nix/store/8javli69jhj3bkql2c35gsj5vl91p382-linux-4.19.16.tar.xz

1 files changed, 1 insertions, 1 deletions

diff --git a/src/nix/progress-bar.hh b/src/nix/progress-bar.hh
index af8eda5a8..4d61175c2 100644
--- a/src/nix/progress-bar.hh
+++ b/src/nix/progress-bar.hh
@@ -4,7 +4,7 @@
 
 namespace nix {
 
-void startProgressBar();
+void startProgressBar(bool printBuildLogs = false);
 
 void stopProgressBar();