Add `xmlsec` package to flake.nix (#15532)

changelog.d/15532.misc

@@ -0,0 +1 @@
+Install the `xmlsec` package and switch back to the upstream [cachix/devenv](https://github.com/cachix/devenv) repo in the nix development environment.

flake.nix

@@ -93,6 +93,7 @@
 
                   # Native dependencies for unit tests (SyTest also requires OpenSSL).
                   openssl
+                  xmlsec
 
                   # Native dependencies for running Complement.
                   olm