fix linux bootTime

[m4rch3n1ng]

the previous implementation was adding the elapsed time since boot to the current time instead of subtracting it.

previous implementation:

$ fastfetch
    Uptime: 8 days, 13 hours, 48 mins
$ fastfetch --uptime-format '{6}'
    Uptime: Uptime: 2024-09-13 17:16:04

where the bootTime (--uptime-format '{6}') is ~8 days in the future

new implementation:

$ fastfetch
    Uptime: 8 days, 13 hours, 52 mins
$ fastfetch --uptime-format '{6}'
    Uptime: 2024-08-27 13:34:13

see also

fastfetch/src/detection/uptime/uptime_windows.c

Line 9 in 8371920

result->bootTime = ffTimeGetNow() - result->uptime;

[CarterLi]

oops. God knows what I was thinking about.