`self.spawn` is a blocking function but is run on `async` threads

[weissi]

swift-subprocess/Sources/Subprocess/Configuration.swift

Line 69 in 80bd50f

let spawnResults = try self.spawn(

execve/posix_spawn will block the child process (which is fine) but we also need to communicate information back into the parent process (to figure out if execve worked). That however may block, for example if we spawn an executable on a network'd file system. But in the code today, try self.spawn(...) is run on async threads which is a problem as it might block all asynchronous work from happening potentially indefinitely.

self.spawn should be offloaded onto a thread pool where blocking is okay.