Fixed pfw install command on Linux

[thinkpractice]

Hi @stephencelis and @mbrandonw ,

as promised a PR with two minor changes that makes pfw install work on Linux too. Before, pfw install would crash with the following message:

Object 0x778a2400f3f0 of class _MultiHandle deallocated with non-zero retain count 2. This object's deinit, or something called from it, may have created a strong reference to self which outlived deinit, resulting in a dangling reference.

Apparently (and according to Codex), this is a common issue with URLSession on Linux. From my own experience I can confirm there are large differences between URLSession on Apple platforms and the open source version on Linux. The latter uses libcurl and doesn't implement all APIs that are available on Apple platforms.

I hope this (minor) fix is acceptable also for Apple platforms. I still have one warning compiling:

warning: 'zipfoundation': found 1 file(s) which are unhandled; explicitly declare them as resources or exclude from the target

If that's a blocking issue for you I can investigate.

kind regards,

Tim

[stephencelis]

Thanks!