cli: read "paths" from stdin for commands that take them

[nrdxp]

Is your feature request related to a problem? Please describe.
If the user wants to pass a list of paths to say nix path-info or nix-build, they run the risk of recieving a "too many args" OS error.

Since passing derivations around can be very efficient when handled in batches it would be nice not to have a cap on this. The current workaround is xargs, which I guess is fine, but not exactly intuitive or necessarily efficient.

Describe the solution you'd like
I could see it two ways, if the command recieves a non-empty stdin, simply add it to whatever list of paths already exist in the cli, or we could make it explicit and require something like nix-build - to specify reading paths from stdin.

Describe alternatives you've considered
Basically just keep using xargs, but even if the arg limit is not an issue, shell pipelines are far more readable than nested process substititions, e.g:

$ which svn | nix-store -qR | nix-store --realize
# is nicer than
$ nix-build $(nix-store -qR $(which svn))

Additional context
Hit this a couple of times when trying to ensure the entire build closure is cached by parsing the results of a --dry-run: divnix/std-action/run/build.sh#14

[nrdxp]

Just found nix-store --export and it's probably the most important usecase for this, since using xargs wouldn't wok in this case to serialise a potentially large list of derivations at once to another store.