testVersion: when there is a error show the whole error

before

$ nix build ".#whipper.tests.version"
whipper>   File "/nix/store/2iiyy58pmm1ys6dy8ycbmmmfm67iakv1-whipper-0.10.0/bin/.whipper-wrapped", line 6, in <module>
whipper>   File "/nix/store/2iiyy58pmm1ys6dy8ycbmmmfm67iakv1-whipper-0.10.0/lib/python3.9/site-packages/whipper/__init__.py", line 5, in <module>
note: keeping build directory '/tmp/nix-build-whipper-0.10.0-test-version.drv-7'

after
$ nix build ".#whipper.tests.version"
whipper> Traceback (most recent call last):
whipper>   File "/nix/store/2iiyy58pmm1ys6dy8ycbmmmfm67iakv1-whipper-0.10.0/bin/.whipper-wrapped", line 6, in <module>
whipper>     from whipper.command.main import main
whipper>   File "/nix/store/2iiyy58pmm1ys6dy8ycbmmmfm67iakv1-whipper-0.10.0/lib/python3.9/site-packages/whipper/__init__.py", line 5, in <module>
whipper>     from pkg_resources import (get_distribution,
whipper> ModuleNotFoundError: No module named 'pkg_resources'
note: keeping build directory '/tmp/nix-build-whipper-0.10.0-test-version.drv-34'
error: builder for '/nix/store/5lxjicdhwgmjcz9ddlxgq3s3gyaa6lz4-whipper-0.10.0-test-version.drv' failed with exit code 1;

1 files changed, 6 insertions, 2 deletions

diff --git a/pkgs/build-support/trivial-builders.nix b/pkgs/build-support/trivial-builders.nix
index 4a3d3778881..c5853ea664d 100644
--- a/pkgs/build-support/trivial-builders.nix
+++ b/pkgs/build-support/trivial-builders.nix
@@ -810,7 +810,11 @@ rec {
       command ? "${package.meta.mainProgram or package.pname or package.name} --version",
       version ? package.version,
     }: runCommand "${package.name}-test-version" { nativeBuildInputs = [ package ]; meta.timeout = 60; } ''
-      ${command} |& grep -Fw ${version}
-      touch $out
+      if output=$(${command} 2>&1); then
+        grep -Fw "${version}" - <<< "$output"
+        touch $out
+      else
+        echo "$output" >&2 && exit 1
+      fi
     '';
 }