Ark --install command panics

[jmcphers]

To repro, just run ark --install. This appears:

thread 'main' panicked at /home/jmcphers/git/ark/crates/harp/src/command.rs:33:42:
called `Result::unwrap()` on an `Err` value: NotPresent
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Some debugging suggests the problem is that this command requires R_HOME to be set. So you can do this:

export R_HOME=$(R RHOME)

After this you can install, but you get a spurious error about needing a connection file.

./ark --install
Successfully installed Ark Jupyter kernelspec.

    Kernel: /home/jmcphers/.local/share/jupyter/kernels/ark/kernel.json
    
Error: A connection file must be specified. Use the `--connection_file` argument.

It's probably fine to require R_HOME to be set so that Ark is registered against the right version of R, but we should check and see if it's set and show something friendly if it isn't.

[fithisux]

Same on windows 10 x64, the kernel appears in Jupyter lab but cannot run anything.

But R is in the PATH and R_HOME is set correctly. @jmcphers

PATH is c:\winoss\R\bin

IRKernel works fine in Jupyter lab.

Positron works fine on the other hand