stalwart_0_16: configure openssl installation

`openssl-sys` attempts to dynamically find system `openssl` installation
via `pkg-config` but fails on darwin systems. This is _likely_ due to
sandbox profile however I cannot replicate the hydra failure locally.

Standardizing it across all platforms via the `OPENSSL_*` environment
variables precludes the need for `pkg-config` to dynamically resolve
`openssl`.

Resolves: #535421

pkgs/by-name/st/stalwart_0_16/package.nix

@@ -64,7 +64,10 @@ rustPlatform.buildRustPackage (finalAttrs: {
   cargoHash = "sha256-HVS89Wtjb2nIdyygP8bPRbVhyMRnJlZDfoCQqiMdVe0=";
 
   env = {
+    # https://docs.rs/openssl/latest/openssl/#manual
     OPENSSL_NO_VENDOR = true;
+    OPENSSL_DIR = lib.getDev openssl;
+    OPENSSL_LIB_DIR = "${lib.getLib openssl}/lib";
     ZSTD_SYS_USE_PKG_CONFIG = true;
     ROCKSDB_INCLUDE_DIR = "${rocksdb}/include";
     ROCKSDB_LIB_DIR = "${rocksdb}/lib";