Fix building debian packages on non-clean checkouts (#17390)

If we leave the `.so` in place it causes the tests to fail, as it gets
picked up (instead of the newly built .so) and so fails with mismatched
GLIBC errors.

changelog.d/17390.misc

+Fix building debian packages on non-clean checkouts.

docker/build_debian.sh

@@ -11,6 +11,9 @@ DIST=$(cut -d ':' -f2 <<< "${distro:?}")
 cp -aT /synapse/source /synapse/build
 cd /synapse/build
 
+# Delete any existing `.so` files to ensure a clean build.
+rm -f /synapse/build/synapse/*.so
+
 # if this is a prerelease, set the Section accordingly.
 #
 # When the package is later added to the package repo, reprepro will use the