[vcpkg] Fix warning on Apple

[mcraveiro]

This patch addresses a warning when compiling on OSX (I used LLVM Clang 7.0). The warning is as follows:

[12/64] Building CXX object CMakeFiles/vcpkg.dir/src/vcpkg/base/system.cpp.o
../src/vcpkg/base/system.cpp:34:52: warning: result of comparison of constant -1 with expression of type 'bool' is always true [-Wtautological-constant-out-of-range-compare]
        Checks::check_exit(VCPKG_LINE_INFO, result != -1, "Could not determine current executable path.");
                                            ~~~~~~ ^  ~~
../src/vcpkg/base/system.cpp:36:52: warning: result of comparison of constant -1 with expression of type 'bool' is always true [-Wtautological-constant-out-of-range-compare]
        Checks::check_exit(VCPKG_LINE_INFO, result != -1, "Could not determine current executable path.");
                                            ~~~~~~ ^  ~~
2 warnings generated.

The warning is the result of using bool as the return type of _NSGetExecutablePath, when the documentation states it is an int [1]:

bool result = _NSGetExecutablePath(buf, &size);

The patch sets result to match the return type of the function. It was tested using LLVM Clang 7.0 and compiles cleanly.

[1] https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/dyld.3.html

[ras0219-msft]

Thanks for the fix and thanks @zhihaoy for the review 👍 !