liquibase: Add sourceRoot

To fix the build issue that was introduced in f373abe
when the custom `unpackPhase` was removed without correctly validating the build,
which results in:
```
error: builder for '/nix/store/5qax6r6kxi4spn5bc7qjpgglxb9kvhxp-liquibase-4.29.2.drv' failed with exit code 1;
       last 3 log lines:
       > Running phase: unpackPhase
       > unpacking source archive /nix/store/v12lcsjxsq3n4lgi8za3k9mqc8pak59b-liquibase-4.29.2.tar.gz
       > unpacker produced multiple directories
       For full logs, run 'nix log /nix/store/5qax6r6kxi4spn5bc7qjpgglxb9kvhxp-liquibase-4.29.2.drv'.
```

pkgs/development/tools/database/liquibase/default.nix

@@ -35,6 +35,8 @@ stdenv.mkDerivation (finalAttrs: {
   nativeBuildInputs = [ makeWrapper ];
   buildInputs = [ jre ];
 
+  sourceRoot = ".";
+
   installPhase =
     let addJars = dir: ''
       for jar in ${dir}/*.jar; do