bugfix: Executable should record return code on CalledProcessError

Spack's `Executable` class isn't properly returning a called process's
return code when it fails with a `CalledProcessError`.  Record it before
raising a `ProcessError` so that client code can query it later.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/spack/util/executable.py

@@ -295,11 +295,11 @@ def streamify(arg, mode):
             raise ProcessError("%s: %s" % (self.exe[0], e.strerror), message)
 
         except subprocess.CalledProcessError as e:
+            self.returncode = e.returncode
             if fail_on_error:
                 raise ProcessError(
                     str(e),
-                    "\nExit status %d when invoking command: %s"
-                    % (proc.returncode, cmd_line_string),
+                    f"\nExit status {e.returncode} when invoking command: {cmd_line_string}",
                 )
         except subprocess.TimeoutExpired as te:
             proc.kill()