Pipelines: Fix issue that allowed failing jobs to pass (#22199)

This is a workaround for an issue with how "spack install" is invoked from within "spack ci rebuild".  The fact that we don't get an exception or even the actual returncode when using the object returned by spack.util.executable.which('spack') to install the target spec means we get no indication of failures about the install command itself.  Instead we rely on the subsequent buildcache creation failure to fail the job.

lib/spack/spack/ci.py

@@ -1292,6 +1292,8 @@ def push_mirror_contents(env, spec, yaml_path, mirror_url, build_id,
                 tty.msg('Permission problem writing to {0}'.format(
                     mirror_url))
                 tty.msg(err_msg)
+            else:
+                raise inst
 
 
 def copy_stage_logs_to_artifacts(job_spec, job_log_dir):