On NixOS, the default `uptime` command from coreutils does not support
the `-p` option.
This replaces the previous `bash`-based solution with a call to
`/proc/uptime`,
and parses the result in JavaScript to produce a human-readable format
like:
"up 1 day, 2 hours, 5 minutes"
This makes uptime reporting fully compatible with NixOS and more
portable.