Building against API 28+ without libandroid-spawn has different nullability for posix_spawn* types

[marcprux]

When building against an Android SDK that targets API level 28 and higher, we no longer need to include the termux libandroid-spawn.so. But now the posix_spawn* types have a different nullability, leading to errors like this one:

/home/runner/work/swift-package-builds/swift-package-builds/swift-tools-support-core/Sources/TSCBasic/Process/Process.swift:736:50: error: value of optional type 'posix_spawn_file_actions_t?' (aka 'Optional<OpaquePointer>') must be unwrapped to a value of type 'posix_spawn_file_actions_t' (aka 'OpaquePointer')
 734 |                 // If no redirect was requested, open the pipe for stderr.
 735 |                 try open(pipe: &stderrPipe)
 736 |                 posix_spawn_file_actions_adddup2(&fileActions, stderrPipe[1], 2)
     |                                                  |- error: value of optional type 'posix_spawn_file_actions_t?' (aka 'Optional<OpaquePointer>') must be unwrapped to a value of type 'posix_spawn_file_actions_t' (aka 'OpaquePointer')
     |                                                  `- note: force-unwrap using '!' to abort execution if the optional value contains 'nil'
 737 | 
 738 |                 // Close the other ends of the pipe since they were dupped to 2.

The nullability difference is mentioned at 52cb792. We can just update swift-tools-support-core and other packages that use posix_spawn to special-case Android's optionality, but is there any way that the code can support both the older 6.1 SDK and future 6.2+ SDKs with the differing nullability?

[finagolfin]

Just modify all such code to use the optional, which is now the official API in Bionic. As I mentioned privately, this 6.1 SDK that still uses the android-spawn backport library is the last one I will build here, switching to testing the official 6.2 and 6.3 SDK bundles in this CI once they're available. It doesn't matter to me if those Swift packages you mention no longer work with this 6.1 SDK.

[finagolfin]

Depending on whether #221 works, I'll look at fixing this.