nix: fix nix-cache builds on MacOS

symlink from /opt/nix requires this setting

Signed-off-by: Jakub Sokołowski <jakub@status.im>

ci/Jenkinsfile.nix-cache

@@ -2,6 +2,8 @@ pipeline {
   agent { label params.AGENT_LABEL }
 
   environment {
+    /* See nix/README.md */
+    NIX_IGNORE_SYMLINK_STORE = 1
     /* we source .bash_profile to be able to use nix-store */
     NIX_SSHOPTS = "-o StrictHostKeyChecking=no source .bash_profile;"
     /* where our /nix/store is hosted */