fix(install): actually return right error codes

Previously exit was called with the stdout content of
the main function. This did not make sense and
resulted in the script not returning the expected
exit codes when main returned a non-zero code.

Strangely enough this broken code was introduced in
response to anchore#1566 but I do not see how it ever worked
unless the script used to echo its intended exit code.

Signed-off-by: Stefan Hacker <[email protected]>

install.sh

@@ -694,6 +694,7 @@ main() (
 set +u
 if [ -z "${TEST_INSTALL_SH}" ]; then
   set -u
-  exit $(main "$@")
+  main "$@"
+  exit $?
 fi
-set -u
+set -u