Revert "Fixed SC2091: Remove surrounding $() to avoid executing outpu…

…t." and fix it another way

This reverts commit 67bcaea. The sub-shell was required to prevent the updated umask from affecting later steps. It broke the permissions of the fetched assets for the control panel: https://discourse.mailinabox.email/t/admin-panel-broken-after-restore-upgrade/12112/24

Instead, the `$()` is replaced with just `()` to create a subshell without executing its output.

setup/management.sh

@@ -54,7 +54,7 @@ hide_output $venv/bin/pip install --upgrade \
 # Create a backup directory and a random key for encrypting backups.
 mkdir -p "$STORAGE_ROOT/backup"
 if [ ! -f "$STORAGE_ROOT/backup/secret_key.txt" ]; then
-	umask 077; openssl rand -base64 2048 > "$STORAGE_ROOT/backup/secret_key.txt"
+	(umask 077; openssl rand -base64 2048 > "$STORAGE_ROOT/backup/secret_key.txt")
 fi