nix build: make dry-run to print a json output if --json is enabled

[picnoir]

Fixes #4747

Before:

./outputs/out/bin/nix build --dry-run --experimental-features nix-command --json -f default.nix don't know how to build these paths (may be caused by read-only store access):
  /nix/store/cm7abdds6mr136s5x59d20z5h4c28adw-nix-2.4pre20210423_fe2bf46.drv

After:

[nix-shell:~/code-root/github.com/NixOS/nix]$ ./outputs/out/bin/nix build --dry-run --experimental-features nix-command --json -f default.nix don't know how to build these paths (may be caused by read-only store access):
  /nix/store/5llx7yykal9jsrj0fkwk3x4fzwy2wahg-nix-2.4pre20210427_7dca9c2.drv
[{"drvPath":"/nix/store/5llx7yykal9jsrj0fkwk3x4fzwy2wahg-nix-2.4pre20210427_7dca9c2.drv","outputs":{"out":"/nix/store/plnsw4ygkgi22rkjxh68ylvyhawwrprs-nix-2.4pre20210427_7dca9c2"}}]