When running a `:b` command in the repl, after building the derivations
query the store for its outputs rather than just assuming that they are
known in the derivation itself (which isn’t true for CA derivations)
Fix#5328
Fill `NIX_CONFIG` with the value of the current Nix configuration before
calling the nix subprocesses in the repl
That way the whole configuration (including the possible
`experimental-features`, a possibly `--store` option or whatever) will
be made available.
This is required for example to make `nix repl` work with a custom
`--store`