fix(secret): skip non-seekable files during scanning

[salome-voltz]

Context

Ggshield crashes on scanning non-seekable files, to reproduce (only on linux):

> cd test_folder/
> ln -s /proc/self/mounts ./mtab
> ls -l mtab #verify if the symbolic link is correctly set up
> ggshield secret scan path ./mtab

Should return:

Scanning... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━   0% 0 / 1

Error: [Errno 22] Invalid argument

Explicitly skip them with a message:

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.94%. Comparing base (f92b323) to head (5681156).
⚠️ Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
ggshield/verticals/secret/secret_scanner.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1133      +/-   ##
==========================================
- Coverage   91.96%   91.94%   -0.02%     
==========================================
  Files         144      144              
  Lines        6174     6196      +22     
==========================================
+ Hits         5678     5697      +19     
- Misses        496      499       +3     

Flag	Coverage Δ
unittests	91.94% <75.00%> (-0.02%)	⬇️

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[agateau-gg]

I think you can remove the error message: it's never shown.

[agateau-gg]

Looks good, thanks!

Can you squash and merge?