devenv: Provide locale archive

Without this, the Iconv tests on CI started to fail in `setLocale` invocation:

    PHPUnit\Framework\Exception: The locale functionality is not implemented on your platform, the specified locale does not exist or the category name is invalid.

    /home/runner/work/transcoder/transcoder/tests/IconvTranscoderTest.php:25

Author: jtojnar

Diff for: flake.nix

@@ -50,6 +50,10 @@
             phpBase.packages.composer
             pkgs.phpactor
           ];
+
+          env = pkgs.lib.optionalAttrs pkgs.stdenv.hostPlatform.isLinux {
+            LOCALE_ARCHIVE = "${pkgs.glibcLocales}/lib/locale/locale-archive";
+          };
         };
       };
 }