Previously the builder scripts defined a readonly `VERSION` environment
variable for the Keyman version. That caused problems when another
(external) script tried to define a `VERSION` variable. We encountered
this problem when trying to move the TC build steps of a configuration
into a single script (#13399) when we tried to source `~/.nvm/nvm.sh`.
This change uses a Keyman specific prefix for the version variables and
renames `VERSION` → `KEYMAN_VERSION` etc. Unfortunately these variables
are used in a lot of places, so this turned out to be a bit of a yak
shave.
Test-bot: skip
Adds run-xcodebuild function to wrap xcodebuild calls to keep logging
and error handling consistent. Currently used only by the iOS build; the
macOS build has a corresponding execBuildCommand function which does
pretty much the same thing (changing that would belong in a separate PR
I think and low priority.)
