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 77.27273% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.90%. Comparing base (f92b323) to head (92cc9f1).

Files with missing lines	Patch %	Lines
ggshield/verticals/secret/secret_scanner.py	25.00%	3 Missing ⚠️
ggshield/core/scan/scannable.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1133      +/-   ##
==========================================
- Coverage   91.96%   91.90%   -0.07%     
==========================================
  Files         144      144              
  Lines        6174     6189      +15     
==========================================
+ Hits         5678     5688      +10     
- Misses        496      501       +5     

Flag	Coverage Δ
unittests	91.90% <77.27%> (-0.07%)	⬇️

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