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

these 2 derivations will be built:
  /nix/store/fja1dsf7xw0m2dkcljjx2avzfr2r564l-install-agenix-shell.drv
  /nix/store/46gmqq06r1q11y9lisq4y2vy1mrzbhsr-nix-shell.drv
nix-shell> source: sourcing removeStubsFromRunpathHook.bash (hostOffset=-1) (targetOffset=0)
nix-shell> source: added removeStubsFromRunpathHookRegistration to prePhases
nix-shell> Sourcing fix-elf-files.sh
nix-shell> Running phase: buildPhase