fix(scripts): use safe temp directories and fix composer exit code

- install-ntfy.sh: use mktemp -d with cleanup trap instead of /tmp/ntfy_*
- install-git-crypt.sh: use mktemp -d with cleanup trap instead of /tmp/git-crypt
- install-composer.sh: only move composer.phar if installation succeeded

scripts/install-composer.sh

@@ -22,5 +22,7 @@ fi
 php composer-setup.php --quiet
 RESULT=$?
 rm composer-setup.php
-mv composer.phar ~/.local/bin/composer
+if [ $RESULT -eq 0 ]; then
+  mv composer.phar ~/.local/bin/composer
+fi
 exit $RESULT