build: Fix cleanup of the backup composer lock file

The trap function was registered as the last bash instruction and as a
result was not executed if anything failed before, e.g. the composer
update command or building the PHAR itself.

In the event the build PHAR script fails, the temporary backup
composer.lock file may be created and not removed.

.gitignore

@@ -6,6 +6,7 @@ dist/
 tests/phpunit.xml
 vendor/
 /composer.lock
+/composer.lock.back
 phpunit.xml
 /.phpunit.result.cache
 /phpcs.xml

build-phar.sh

@@ -9,11 +9,11 @@ function restorePlatform {
     mv -f composer.lock.back composer.lock || true
 }
 
+trap restorePlatform exit
+
 # lock PHP to minimum allowed version
 composer config platform.php 7.2.0
 cp composer.lock composer.lock.back || true
 composer update
 
 php box.phar compile -vv
-
-trap restorePlatform exit