It's not very effective.

E.g. 'hydra-eval-jobs ~/Dev/patchelf' is enough to evaluate patchelf -
no need to set up a $NIX_PATH, pass arguments, etc.

No more need for a reproduction script! It just says something like

  If you have Nix installed, you can reproduce this build on your own
  machine by running the following command:

  # nix build github:edolstra/dwarffs/09c823e977946668b63ad6c88ed358b48220f124:hydraJobs.build.x86_64-linux

May 15 09:20:10 chef hydra-queue-runner[27523]: Hydra::Plugin::GitlabStatus=HASH(0x519a7b8)->buildFinished: Can't call method "value" on an undefined value at /nix/store/858hinflxcl2jd12wv1r3a8j11ybsf6w-hydra-0.1.2629.89fa829/libexec/hydra/lib/Hydra/Plugin/GitlabStatus.pm line 57.

The hydra flake now exports a Nixpkgs overlay and a NixOS module.

Flake input changes:

* Updated 'nix': 'github:NixOS/nix/eb19ff3b82240326fc0e999e09f81b6c8ed98640' -> 'github:NixOS/nix/144bb3ef7ddca6502ea3e1878f7fc0a3f9013aba'

This is now propagated by Nix.

Flake input changes:

* Updated 'nix': 'github:NixOS/nix/144bb3ef7ddca6502ea3e1878f7fc0a3f9013aba' -> 'github:NixOS/nix/6529490cc10018d5191e50c482ac1180b96b1a3c'

This reverts commit 345512a.

Flake input changes:

* Updated 'nix': 'github:NixOS/nix/6529490cc10018d5191e50c482ac1180b96b1a3c' -> 'github:NixOS/nix/4ad5826a182f8eeb4cbc61e27d2128b08050b0d4'

This got lost when I converted from releaseTools.nixBuild to
stdenv.mkDerivation. Doh!

If we don't see machine that supports a build step for
'max_unsupported_time' seconds, the step is aborted. The default is 0,
which is appropriate for Hydra installations that don't provision
missing machines dynamically.

 Abort unsupported build steps

Otherwise, errors will not be shown to end-users, which makes debugging
long evals pretty much impossible.

(cherry picked from commit 76299b9)

Flake input changes:

* Updated 'nix': 'github:NixOS/nix/4ad5826a182f8eeb4cbc61e27d2128b08050b0d4' -> 'github:NixOS/nix/3e7aab81ce6787e01ea4ced5af1fc6a84e523762'
* Updated 'nix/nixpkgs': 'github:edolstra/nixpkgs/43e283790fbe330b723ac44b584824cf6d85c1ee' -> 'github:NixOS/nixpkgs/b88ff468e9850410070d4e0ccd68c7011f15b2be'
* Updated 'nixpkgs': 'github:edolstra/nixpkgs/d959778e0934c4c93896d509a65de798909c7cdd' -> 'github:NixOS/nixpkgs/b0c285807d6a9f1b7562ec417c24fa1a30ecc31a'

Apparentely, buildEnv in 20.03 no longer respects
propagated-build-inputs.

Note that the use of a library function (closePropagation) seems
fundamentally wrong to me - propagated-build-inputs should be used at
runtime, not at evaluation time.

In Nixpkgs 20.03, Mercurial fails if PYTHONPATH is set:

  $ hg
  Traceback (most recent call last):
    File "/nix/store/q7s856v6nw4dffdrm9k3w38qs35i8kr3-mercurial-5.2.2/bin/..hg-wrapped-wrapped", line 37, in <module>
      dispatch.run()
    File "/nix/store/bffdy7q3wi3qinflnvbdkigqj39zzynd-python3-3.7.6/lib/python3.7/importlib/util.py", line 245, in __getattribute__
      self.__spec__.loader.exec_module(self)
    File "<frozen importlib._bootstrap_external>", line 728, in exec_module
    File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
    File "/nix/store/q7s856v6nw4dffdrm9k3w38qs35i8kr3-mercurial-5.2.2/lib/python3.7/site-packages/mercurial/dispatch.py", line 10, in <module>
      import difflib
    File "/nix/store/bffdy7q3wi3qinflnvbdkigqj39zzynd-python3-3.7.6/lib/python3.7/difflib.py", line 1084, in <module>
      import re
    File "/nix/store/bffdy7q3wi3qinflnvbdkigqj39zzynd-python3-3.7.6/lib/python3.7/re.py", line 143, in <module>
      class RegexFlag(enum.IntFlag):
  AttributeError: module 'enum' has no attribute 'IntFlag'