Fix sign job failure: python3 missing in certum-signer container

[Copilot]

The sign job's "Connect SimplySign" step was failing with python3: command not found (exit code 127), aborting the release signing process before it could log in to SimplySign.

Root Cause

• The job runs inside ghcr.io/reactiveui/certum-signer:latest, a signing-focused container image that does not bundle Python 3.
• TOTP generation for the SimplySign login relies on an inline python3 -c '...' script to derive the OTP from CERTUM_OTP_URI.

Fix

• Added a guard in .github/workflows/BuildDeploy.yml before the OTP generation call that installs python3 via apt-get only if it isn't already present in the container.

if ! command -v python3 >/dev/null 2>&1; then
  apt-get update && apt-get install -y --no-install-recommends python3
fi

OTP="$(python3 -c '...')"

This keeps the existing OTP logic untouched while making the step resilient to the container image lacking (or later dropping) Python 3.