Add MinVerVersionOverride property to simplify usage of both MinVer and minver-cli in the same build

[adamralph]

Currently if you want to use both MinVer and minver-cli in the same build, which at least https://github.com/SQLStreamStore/SQLStreamStore.HAL is doing, you have to ensure that the inputs to each of them is identical so that they produce the same version, which could be a little error prone.

With the introduction of this new property, you can run the CLI, and use the version it produces for everything. Just set the MINVERVERSIONOVERRIDE environment variable to the version that comes out of the CLI and the build targets in the MinVer package will just use that value instead of invoking the versioning logic a second time:

// using SimpleExec;
var version = Read("minver");

Environment.SetEnvironmentVariable(
    "MINVERVERSIONOVERRIDE", version, EnvironmentVariableTarget.Process);

Run("dotnet", "pack");

// do other stuff with the version

[adamralph]

/cc @thefringeninja