only link to one jemalloc build

Without setting JEMALLOC_OVERRIDE, we end up linking to two different
jemalloc builds. Once dynamically, as a transitive dependency through
rocksdb, and a second time to the static jemalloc that tikv-jemalloc-sys
builds.

This fixes dynamically-linked jemalloc builds, for the reasons described
in <https://github.com/girlbossceo/conduwuit/pull/400#issue-2316700200>.

nix/shell.nix

@@ -30,5 +30,9 @@ mkShell {
     toolchain
   ]
   ++
-  default.nativeBuildInputs;
+  default.nativeBuildInputs
+  ++
+  default.propagatedBuildInputs
+  ++
+  default.buildInputs;
 }