Fix UB warning on OSX

[Thomas1664]

Related: microsoft/vcpkg#28582 I suspect this is because we have to use mutexes on OSX because their version of popen isn't threat safe.

Fixes /Users/runner/work/1/s/src/vcpkg/dependencies.cpp:715:47: runtime error: load of value <unknown>, which is not a valid value for type 'vcpkg::UnsupportedPortAction'

Enums are treated as their underlying type (bool in this case) which is not default initialized. Warning is the default value of the enum (supposing the compiler is kind enough to zero initialize it), i.e. false.

[Thomas1664]

Confirmed my initial guess: cmd_execute_and_capture_output_parallel takes on OSX 2x longer than on Linux (86 ms on Linux vs. 159 ms on OSX). Unfortunately, we have to keep the mutex (see #695) until OSX fixes their implementation of popen. I didn't report the issue to Apple because I don't know how.

[BillyONeal]

Thanks!