mksbuild should not alway skip updates for debian

[Nick Brown]

'updates' is a valid source for Debian: https://wiki.debian.org/StableUpdates however mksbuild always skips, with no option to include, for debian.

This bit of code appears enforces skipping updates this:
https://git.launchpad.net/ubuntu/+source/ubuntu-dev-tools/tree/mk-sbuild?h=ubuntu%2Fdevel#n675
```
    # Debian only performs security updates
    SKIP_UPDATES=1
    if [ -z "$SOURCES_SECURITY_SUITE" ]; then
        if debian_dist_ge "$RELEASE" "bullseye"; then
            SOURCES_SECURITY_SUITE="RELEASE-security"
        else
            SOURCES_SECURITY_SUITE="RELEASE/updates"
        fi
    fi
```

The updates source should be made available, with standard user option to skip.