Pin NextCloudPi installer to last known-good stable release

The floating "master" branch of nextcloud/nextcloudpi's install.sh
started rejecting our default Debian 12 base with "distro not
supported" (#15944) after a regression landed upstream; the script
is third-party code we don't audit or control. Pin to v1.57.1, the
latest stable (non-prerelease) release, which explicitly targets
Debian bookworm and predates the regression.

install/nextcloudpi-install.sh

@@ -13,10 +13,12 @@ setting_up_container
 network_check
 update_os
 
+NCP_INSTALLER_REF="v1.57.1"
+
 msg_warn "WARNING: This script will run an external installer from a third-party source (https://nextcloudpi.com/)."
 msg_warn "The following code is NOT maintained or audited by our repository."
 msg_warn "If you have any doubts or concerns, please review the installer code before proceeding:"
-msg_custom "${TAB3}${GATEWAY}${BGN}${CL}" "\e[1;34m" "→  https://raw.githubusercontent.com/nextcloud/nextcloudpi/master/install.sh"
+msg_custom "${TAB3}${GATEWAY}${BGN}${CL}" "\e[1;34m" "→  https://raw.githubusercontent.com/nextcloud/nextcloudpi/${NCP_INSTALLER_REF}/install.sh"
 echo
 read -r -p "${TAB3}Do you want to continue? [y/N]: " CONFIRM
 if [[ ! "$CONFIRM" =~ ^([yY][eE][sS]|[yY])$ ]]; then
@@ -25,7 +27,11 @@ if [[ ! "$CONFIRM" =~ ^([yY][eE][sS]|[yY])$ ]]; then
 fi
 
 msg_info "Installing NextCloudPi (Patience)"
-$STD bash <(curl -fsSL https://raw.githubusercontent.com/nextcloud/nextcloudpi/master/install.sh)
+# Pinned to the last known-good stable release instead of the floating "master"
+# branch: master has repeatedly broken distro detection on our default Debian
+# base out from under this script (e.g. #15944), since it's third-party code
+# not audited or version-locked by us.
+$STD bash <(curl -fsSL "https://raw.githubusercontent.com/nextcloud/nextcloudpi/${NCP_INSTALLER_REF}/install.sh")
 msg_ok "Installed NextCloudPi"
 
 motd_ssh