w: Fix formatting of command line value

[alxndrv]

The command line value exposed by the kernel in /proc/<pid>/cmdline represents whitespace as NULL bytes.
Using read_to_string() on this directly will cause the output to join all parts of the original command into one long string.
This PR adds a very simple fix to avoid this.

Before:

$ cargo run -q w
 22:16:22 up  2:00,  1 user,  load average: 0.32, 0.37, 0.38
USER     TTY       LOGIN@   IDLE   JCPU   PCPU  WHAT
alx      tty1      20:16    1:59m  0.00   0     /bin/sh/usr/bin/niri-session-l

After:

$ cargo run -q w
 22:17:02 up  2:00,  1 user,  load average: 0.67, 0.46, 0.41
USER     TTY       LOGIN@   IDLE   JCPU   PCPU  WHAT
alx      tty1      20:16    2:00m  0.00   0     /bin/sh /usr/bin/niri-session -l 

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.18%. Comparing base (ee98c0a) to head (5aaa4cd).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/w/src/w.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #666      +/-   ##
==========================================
- Coverage   19.19%   19.18%   -0.01%     
==========================================
  Files          68       68              
  Lines       10135    10139       +4     
  Branches      545      545              
==========================================
  Hits         1945     1945              
- Misses       8190     8194       +4     

Flag	Coverage Δ
macos_latest	6.74% <ø> (ø)
ubuntu_latest	19.54% <0.00%> (-0.01%)	⬇️
windows_latest	0.02% <ø> (ø)

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[Bluemangoo]

Can you fix the tests?

[alxndrv]

Tests are fixed now, but I'm a bit confused by the coverage check. Is there anything I can do to make it come up as successful?