Add parallelism auto-detection from GCS via signed URLs - #620

these 6 derivations will be built:
  /nix/store/wiir95slxlmisxbip6np3k5qbyvlxp72-psyche-workspace-deps-0.2.0.drv
  /nix/store/4lrk40h3kwdxy127a1byws0qr3ck0bky-psyche-python-extension-0.2.0.drv
  /nix/store/frqvzav4jh1wwpq34kb8b79hs7ix38dq-psyche.drv
  /nix/store/cj5wgscgwnvkgg0m6pbpxw77vxic0bc8-psyche-python-env.drv
  /nix/store/fja1dsf7xw0m2dkcljjx2avzfr2r564l-install-agenix-shell.drv
  /nix/store/78vbq15rmlhi58h0kmpbxk5wlrz3hr71-nix-shell.drv