Fix thread unsafety due to internal use of set_var()

[delan]

this is the first of two patches aimed at addressing thread safety issues. set_debug() and set_pipefail() change the debug and pipefail settings by using std::env::set_var(), which is now known to be unsafe to use in multi-threaded programs. note that the unsafety is not because it has global effect when it works as intended (this will be addressed in the second patch), but rather that it may not work as intended if other threads are reading from the environment.

this patch reworks set_debug() and set_pipefail() to write to a LazyLock<AtomicBool> variable for that setting. these variables read from the environment variables on first use. this is technically a breaking change, because consumers may have relied on changing these settings with std::env::set_var(), but such consumers were incorrect anyway.

i’ve also updated the docs for thread safety issues and global state to improve their accuracy.