src/util: Fix compatibility with glibc 2.33 and higher

[vmihalko]

From man 3 errno: The value in errno is significant only when the
return value of the call indicated an error.

Any code using this failed with: realpath failed: Invalid argument

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[codecov]

Codecov Report

Merging #6033 (0bfda95) into develop (eea16dd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6033   +/-   ##
========================================
  Coverage    74.11%   74.11%           
========================================
  Files         1444     1444           
  Lines       157389   157389           
========================================
  Hits        116646   116646           
  Misses       40743    40743           

Impacted Files	Coverage Δ
src/util/file_util.cpp	76.78% <100.00%> (ø)
src/util/tempdir.cpp	80.64% <100.00%> (ø)

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