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

these 20 derivations will be built:
  /nix/store/1jmnm1szi82pmz75h6qw65p38r8y6y43-psyche-workspace-deps-0.2.0.drv
  /nix/store/0wzx5z9m32hmsd235sw75jddiksjc6nw-psyche-centralized-local-testnet-0.2.0.drv
  /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/pcna8hvczs84mqbb1wk5fxf1yka1n0fd-psyche-sidecar-0.2.0.drv
  /nix/store/wg6prrjk1msmqiji7kq714iwl3y5bi08-psyche-sidecar.drv
  /nix/store/21wrfa53cd73iyp67x62nhaw053fd5sa-psyche-sidecar-nixgl-wrapped.drv
  /nix/store/3dyx240cji5my8vhbkvvi5glb6fb26ng-psyche-centralized-server-0.2.0.drv
  /nix/store/6h3fhvpckxkq30bvc20cv56aj6y3m3rd-psyche-solana-client-0.2.0.drv
  /nix/store/3wr4hva4wky85nxqqq4fqw2wr08vzynf-psyche-solana-client.drv
  /nix/store/gghb4rrwrldfjhdpl63jc5znxlb59lgq-psyche-centralized-client-0.2.0.drv
  /nix/store/qhx18r3fkpan3mi7y7vryn0fy0xhij92-psyche-centralized-client.drv
  /nix/store/c5vn7hbp0qvrf6nb3mdqqh7fkqifhqib-psyche-centralized-client-nixgl-wrapped.drv
  /nix/store/swl88b8f2g0c918mz05329bkmw843r2q-psyche-solana-client-nixgl-wrapped.drv
  /nix/store/808y3ssqgfp1lrfhyiwaar3bswj35cg1-psyche-book.drv
  /nix/store/5j6jvpqwxngaxzpq96sbmfj12dim54q5-Caddyfile.drv
  /nix/store/p3jwxnmbmhxj6yi2smx6j13rwgaza2kp-Caddyfile-formatted.drv
  /nix/store/a2ra5v82w873k5hf3cw9ymavpss022q4-validate-psyche-http-docs-caddyfile.drv
validate-psyche-http-docs-caddyfile> 2026/03/06 15:47:49.472	INFO	using config from file	{"file": "./Caddyfile"}
validate-psyche-http-docs-caddyfile> 2026/03/06 15:47:49.476	INFO	adapted config to JSON	{"adapter": "caddyfile"}
validate-psyche-http-docs-caddyfile> Valid configuration