avoid masking exit code of shell commands

When a variables is declared (with local or export) a failing command is ignored[0]. Changing that so abuild should now error out properly in more cases.

Also remove 1 pipe that could have prevented an issue recently[1] where the du command core dumped but abuild didn't error.

The other pipes were not changed for now since it may be better to use set -o pipefail instead. That may require some changes with commands like grep which use a non-zero exit code to indicate if a match was found or similar[2].