add option to disable retry on transient build failure

Start in src/hydra-queue-runner/build-remote.cc at the BuildResult::TransientFailure branch shown in the issue. Verify how the derivation environment is read and confirm that __hydraRetry=false stops retries after the first transient failure while the default behavior remains unchanged.

When Nix returns BuildResult::TransientFailure, Hydra retries the build many times with exponential back-off. This is appropriate when the derivation is simply downloading from the internet, but Nix actually returns this error code quite liberally, and it does it systematically for fixed-output and impure derivations when they fail.

This behavior is sometimes very unwelcome; for example, I am using dedicated fixed-output derivations to run additional, complex tests that involve hardware-in-the-loop and Windows VMs.

The simple patch below allows retries to be disabled for particular derivations in this manner:

    foo = pkgs.stdenv.mkDerivation {
      outputHashAlgo = "sha256";
      outputHashMode = "recursive";
      outputHash = "...";
      __hydraRetry = false;
      ...
    }

When the build of that derivation fails, Hydra immediately marks it as failed after the first attempt.

commit cbdd0167ac1dbd1bfb96a7d7cdeefe01af8599cd (HEAD -> patched-npm, origin/patched-npm)
Author: Sebastien Bourdeauducq <sb@m-labs.hk>
Date:   Thu Mar 14 17:45:32 2019 +0800

    add option to disable retries on transient failures

diff --git a/src/hydra-queue-runner/build-remote.cc b/src/hydra-queue-runner/build-remote.cc
index 69c430eb..bdbc808d 100644
--- a/src/hydra-queue-runner/build-remote.cc
+++ b/src/hydra-queue-runner/build-remote.cc
@@ -344,7 +344,7 @@ void State::buildRemote(ref<Store> destStore,
                     break;
                 case BuildResult::TransientFailure:
                     result.stepStatus = bsFailed;
-                    result.canRetry = true;
+                    result.canRetry = get(step->drv.env, "__hydraRetry", "1") == "1";
                     result.errorMsg = "";
                     break;
                 case BuildResult::TimedOut: