cmake: use absolute path when applying exclude regex

[lupin012]

After #1885 I started to experience linking errors on my machine for targets snapshots and db_toolbox. The problem was caused by library silkworm_node being empty, which in turn was due to a combination of two things:

• one of my Silkworm repo stays in external disk and contains db folder in parent path (i.e. /db/silkworm/...)
• the exclude regex application in silkworm_library CMake function excluded all source files for such weird path

I've added SILKWORM_SRC_DIR in each exclude regex token to filter using absolute paths.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.24%. Comparing base (7b9c76c) to head (ee5ef88).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1902      +/-   ##
==========================================
+ Coverage   52.22%   52.24%   +0.01%     
==========================================
  Files         607      607              
  Lines       45554    45554              
  Branches     7034     7034              
==========================================
+ Hits        23792    23800       +8     
+ Misses      19680    19676       -4     
+ Partials     2082     2078       -4     

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