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
This change fixes a problem if both `master` and a branch added the same
symbol with different version numbers. This causes a merge conflict which
previously caused the API checks to output an error. However, if the
version number in the .symbols file got updated since the base then
it's no error which this change implements.
