Fix race conditions in temporary file creation

Replace weak random number generation with atomic counters and implement
retry loops to handle EEXIST errors in four locations:

1. optimise-store.cc: Fix race in temporary hard link creation

   • Replace rand() with makeTempPath() for unique naming
   • Add infinite retry loop for handling concurrent processes
   • Use std::filesystem for portable path construction

2. indirect-root-store.cc: Fix race in temporary symlink creation

   • Replace rand() with makeTempPath() for unique naming
   • Add infinite retry loop for makeSymlink()
   • Properly handle cleanup of failed temporary symlinks

3. local-store.cc: Fix race in build log creation

   • Replace simple getpid() with makeTempPath() for unique naming
   • Add retry loop for handling concurrent log writes
   • Accept EEXIST as success when another process creates the log

4. local-binary-cache-store.cc: Fix race in cache file creation

   • Replace getpid() + counter with makeTempPath() for unique naming
   • Ensures atomic file updates in binary cache operations

All fixes use the existing makeTempPath() utility which provides atomic
counter + getpid() based unique name generation. All locations ensure
proper SysError exceptions are thrown instead of propagating
std::filesystem errors.

Motivation

Context

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