cksum: fix reading from closed file after vlang/v#25665

[gechandesu]

Part of vlang/v#26096 fix.

[gechandesu]

Hm, CI failure is unrelated... Looks like a many tools need to be fixed

[JalonSolov]

shuf is the only thing failing on Linux. Windows is always... problematic. Someone with a Windows system will need to debug those failures.

At any rate, you are correct that the CI failures are unrelated to your changes.

[gechandesu]

On my Linux machine (Arch Linux) 5 fails now:

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To reproduce just failure 1 run:    '/home/ge/.vlang/v'  '/home/ge/Code/nvcoreutils/src/arch/arch_test.v'
To reproduce just failure 2 run:    '/home/ge/.vlang/v'  '/home/ge/Code/nvcoreutils/src/env/env_nix_test.v'
To reproduce just failure 3 run:    '/home/ge/.vlang/v'  '/home/ge/Code/nvcoreutils/src/id/id_nix_test.v'
To reproduce just failure 4 run:    '/home/ge/.vlang/v'  '/home/ge/Code/nvcoreutils/src/shuf/shuf_test.v'
To reproduce just failure 5 run:    '/home/ge/.vlang/v'  '/home/ge/Code/nvcoreutils/src/stat/stat_nix_test.v'
Summary for all V _test.v files: 5 failed, 42 passed, 47 total. Elapsed time: 10112 ms, on 15 parallel jobs. Comptime: 21877 ms. Runtime: 36841 ms.

[JalonSolov]

Hmm... I get the same 5 failures here (Manjaro Linux). Always more to do...

One of the failures is because the Arch coreutils package doesn't include the arch command. :-\