signal-desktop: Fix the database encryption by preloading SQLCipher

AFAIK this is the only reliable way for us to ensure SQLCipher will be
loaded instead of SQLite. It feels like a hack/workaround but according
to the SQLCipher developers [0] "this issue can and should be handled
downstream at the application level: 1. While it may feel like a
workaround, using LD_PRELOAD is a legitimate approach here because it
will substitute the system SQLite with SQLCipher which is the intended
usage model;".

This fixes #108772 for NixOS 20.09 users who upgrade to NixOS 21.05 and
replaces #117555.

For nixos-unstable users this will unfortunately break everything again
so we should add a script to ease the transition (in a separate commit
so that we can revert it for NixOS 21.05).

[0]: https://github.com/sqlcipher/sqlcipher/issues/385#issuecomment-802874340

nixos/tests/signal-desktop.nix

@@ -44,12 +44,11 @@ import ./make-test-python.nix ({ pkgs, ...} :
     # - https://github.com/NixOS/nixpkgs/issues/108772
     # - https://github.com/NixOS/nixpkgs/pull/117555
     print(machine.succeed("su - alice -c 'file ~/.config/Signal/sql/db.sqlite'"))
-    # TODO: The DB should be encrypted and the following should be machine.fail
-    # instead of machine.succeed but the DB is currently unencrypted and we
-    # want to notice if this isn't the case anymore as the transition to a
-    # encrypted DB can cause data loss!:
     machine.succeed(
-        "su - alice -c 'file ~/.config/Signal/sql/db.sqlite' | grep -i sqlite"
+        "su - alice -c 'file ~/.config/Signal/sql/db.sqlite' | grep 'db.sqlite: data'"
+    )
+    machine.fail(
+        "su - alice -c 'file ~/.config/Signal/sql/db.sqlite' | grep -e SQLite -e database"
     )
   '';
 })