Replace getcwd() with boinc_getcwd()

[computezrmle]

Compiling vboxwrapper with warnings enabled prints this:

vboxwrapper.cpp: In function ‘int main(int, char**)’:
vboxwrapper.cpp:511:11: warning: ignoring return value of ‘char* getcwd(char*, size_t)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  511 |     getcwd(path, sizeof(path));
      |     ~~~~~~^~~~~~~~~~~~~~~~~~~~

To get rid of the warning replace getcwd() with boinc_getcwd().

[Copilot]

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