replaceValidPath should use atomic RENAME_EXCHANGE on Linux

[Mic92]

nix/src/libstore/build/derivation-goal.cc

Line 800 in 26c3fc1

void replaceValidPath(const Path & storePath, const Path & tmpPath)

currently uses a temporary location as a way to recover from interrupts when replacing old store paths with new repaired ones.
However on Linux this can be made more atomic using renameat2() with the RENAME_EXCHANGE flag.
This should leave no time window where the store path does not exist.