maint(resources): `isTestBuild`/`IS_TEST_BUILD` flags are not clearly specified in build triggers

We have three different kinds of builds:

1. Test builds on PR branches
2. Test builds on master/beta/stable-x.y (no PRs associated)
3. Release builds on master/beta/stable-x.y branches

Each of these builds have slightly different characteristics. Currently, it isn't clear if `isTestBuild` covers both (1) and (2) above, or just (1) -- and the documentation is inconsistent. We should clarify this and make usage consistent, whether by eliminating the variable (and using e.g. [`KEYMAN_VERSION_TAG`](https://github.com/keymanapp/keyman/blob/bae371bb50f3c815d5f0ba7ad1868b0fa7191d60/resources/build/builder-basic.inc.sh#L10-L11))