fix(developer): kmcomp package compiler race condition

Fixes a race condition when compiling multiple packages, where the
temp path may be re-used before the compiler has finished with it.

To take advantage of this fix, I wanted to make sure I was using the
right kmcomp.exe when building unit tests for Keyman Core. This
uncovered a couple of other things:

* Updates Keyman Core build infrastructure to use the current kmcomp.exe
  rather than a system-installed one.
* Corrects a bug that was identified by the 15.0 version of kmcomp in
  one unit test (duplicate `&NAME` stores -- the meson test script
  prepends a `&NAME` store to each source file during build).

@keymanapp-test-bot skip

auto: increment beta version to 15.0.257

chore(android,ios,mac): Update crowdin strings for Spanish (Latin America)

auto: increment beta version to 15.0.256

fix(web): osk shrinkage from rounding

auto: increment beta version to 15.0.255

change(ios): modernization of the Keyman app main screen's layout logic

feat(ios): adds back and forward navigation buttons for in-app help

fix(ios): keyboard height on app load

fix(ios): missed i18n site

auto: increment beta version to 15.0.254

chore(ios): FV certificate hash

auto: increment beta version to 15.0.253

fix(ios): fixes OSK mispositioning bug in iOS 15

auto: increment beta version to 15.0.252