Teach installShellCompletion how to install completions from a named
pipe. Also add a convenience flag `--cmd NAME` that synthesizes the name
for each completion instead of requiring repeated `--name` flags.
Usage looks something like
installShellCompletion --cmd foobar \
--bash <($out/bin/foobar --bash-completion) \
--fish <($out/bin/foobar --fish-completion) \
--zsh <($out/bin/foobar --zsh-completion)
Fixes#83284
7e1e8543fc