Add support for environment variables for default values for MinVer settings

[augustoproiete]

This PR adds the ability to use environment variables to set default variables to Cake.MinVer settings. The environment variable is only used for settings that have not been explicitly set in the MinVerSettings in the Cake build script.

Environment variables that translate to MinVer arguments

Environment Variable		MinVerSettings
MINVERAUTOINCREMENT	➡️	AutoIncrement
MINVERBUILDMETADATA	➡️	BuildMetadata
MINVERDEFAULTPRERELEASEPHASE	➡️	DefaultPreReleasePhase
MINVERMINIMUMMAJORMINOR	➡️	MinimumMajorMinor
MINVERTAGPREFIX	➡️	TagPrefix
MINVERVERBOSITY	➡️	Verbosity

Environment variables that change Cake.MinVer behavior

Environment Variable		MinVerSettings
MINVERPREFERGLOBALTOOL	➡️	PreferGlobalTool
MINVERNOFALLBACK	➡️	NoFallback
MINVERTOOLPATH	➡️	ToolPath

---

e.g.

Given the following environment variables have been set:

Environment Variable	Value
MINVERBUILDMETADATA	abcd
MINVERDEFAULTPRERELEASEPHASE	beta

And the Cake script executes:

var buildVersion = MinVer(s => s.WithDefaultPreReleasePhase("preview"));
Information($"Version: {buildVersion.Version}"); // e.g. 1.2.3-preview.0.1+abcd

Will have the same outcome as:

var buildVersion = MinVer(s => s.WithDefaultPreReleasePhase("preview").WithBuildMetadata("abcd"));
Information($"Version: {buildVersion.Version}"); // e.g. 1.2.3-preview.0.1+abcd

The MINVERBUILDMETADATA environment variable is used to automatically set the BuildMetadata setting, and the MINVERDEFAULTPRERELEASEPHASE environment variable is ignored because DefaultPreReleasePhase set explicitly.