Task/bugfix: Ensure that builds from the master branch carry version numbers that are higher than the latest release along the corresponding release branch (r1.5 as of the writing of this ticket).

Background:

As reported by shane.harvey@mongodb.com in Slack, the artifacts generated by Evergreen builds of the libmongocrypt master branch carry a version number that is lower than 1.5.0, which is the current latest release tag on the r1.5 branch. This prevents the Python team from specifying 1.5.0 as the minimum libmongocrypt version because current builds from master are being assigned versions along the lines of 1.5.0-pre3+..., which sort lower than 1.5.0 in SemVer comparisons.

This results from a combination of two factors. Since the 1.3.x releases the libmongocrypt project has begin using release branches (e.g., r1.3, r1.4, r1.5) rather than making releases directly from the master branch. The first is that while some pre-release tags for 1.3, 1.4, and 1.5 were tagged on the master branch, the first full release in each series marked the start of the aforementioned release branches. Second, the Python team did not begin their work until the 1.5 release cycle, so the fact that master builds were being assigned lower version numbers following a final release went unnoticed.