Convert archiver's force_dir_scan variable to an atomic variable.
Commit
bd5132db55 introduced new atomic read/write functions with
full barrier semantics, which are intended to simplify converting
non-performance-critical code to use atomic variables. This commit
demonstrates one such conversion.
Reviewed-by: Yong Li
Discussion: https://postgr.es/m/
20231110205128.GB1315705%40nathanxps13