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

[mcdurdin]

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)