feat: use process-file to allow eval-buffer to work over tramp

[DinoChiesa]

Description

Modify jsonnet-eval-buffer to use process-file, instead of call-process

Motivation and Context

jsonnet-eval-buffer previously used call-process. This works for synchronous, local processes.
When emacs is visiting a .jsonnet file over TRAMP, for example with a filespec like /ssh:remote-machine:~/dev/myfile.jsonnet, call-process invokes jsonnet locally. This fails: the jsonnet program cannot access or read the remote file.

By moving to process-file, the jsonnet program runs on the remote machine and can access the remote file. To make this work, jsonnet must be installed and available on the PATH of the remote machine.

Have you written tests for your change?

No tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project (see https://github.com/bbatsov/emacs-lisp-style-guide).
• My change requires NO change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.