Add support to overwrite version using VERSIONINGIT_PRETEND_VERSION

[basnijholt]

I frequently encountered situations where I need to install a package without having the full version history available. For example when installing a single package from a large monorepo in a Docker image.

This PR adds support to overwrite the version using an environment variable VERSIONINGIT_PRETEND_VERSION, just like setuptools_scm: https://setuptools-scm.readthedocs.io/en/latest/overrides/#pretend-versions.

@jwodder, let me know what you think and then if you are open to this idea we can add docs.

[jwodder]

I appreciate your contribution, but as I said here, using an envvar to set the version for any package being built would result in the version also being overridden for dependencies of that package that also use versioningit & need to be built from source, and that's an inherently broken design that I don't want to implement.