make the gitinfo caches location configurable to be able to reuse the same information over several projects that are built

[tisis2]

i would like to use GitInfo to version all DLLs in my solution to the same version. i use SDK style projects and would like to use the generated assembly. therefore I placed a central Build.Directory.targets for the solution that would add the following:

    <ItemGroup>
        <PackageReference Include="GitInfo" Version="2.2.0">
            <PrivateAssets>all</PrivateAssets>
            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
        </PackageReference>
    </ItemGroup>

    <Target Name="SetVersionFromGitInfo" DependsOnTargets="GitVersion" BeforeTargets="GetAssemblyVersion">
        <PropertyGroup>
            <BaseVersion>$(GitBaseVersionMajor).$(GitBaseVersionMinor).$(GitBaseVersionPatch).$(GitCommits)</BaseVersion>

            <!-- for TFS build GitBranch is not set so we need to get the branch from the TFS variable -->
            <FeatureBranch>$(BUILD_SOURCEBRANCH)</FeatureBranch>
            <FeatureBranch Condition="$(FeatureBranch.StartsWith('refs/heads/'))">$(FeatureBranch.Substring(11))</FeatureBranch>
            <!-- if variable is not set its a local build -->
            <FeatureBranch Condition="$(FeatureBranch) == ''">$(GitBranch)</FeatureBranch>

            <ExtendedVersion>$(BaseVersion) / $(GitCommit) ($(GitIsDirty)) $(FeatureBranch)</ExtendedVersion>

            <Version>$(BaseVersion)</Version>
            <InformationalVersion>$(ExtendedVersion)</InformationalVersion>
        </PropertyGroup>
    </Target>

i recognized that the build time for the round 30 projects increased about 2 minutes after that change.
so i tried to see how to optimize that a bit. we found that the GitInfo for a single project would be cached so we thought it might be a nice approach to move the cache to the solution directory and reuse it so that not every project is recreating its own cache with the same content.

to test that i added the following to the central .targets file:

    <Target Name="OverrideGitInfoCache" BeforeTargets="$(GitInfoDependsOn)">
        <PropertyGroup>
            <_GitInfoFile>../GitInfo.cache</_GitInfoFile>
            <_GitIsDirtyFile>../GitIsDirty.cache</_GitIsDirtyFile>
        </PropertyGroup>
    </Target>

it almost worked. i just recognised that now the cache is still updated on each project, since the is dirty cache was updated all the time and therefore the gitinfo cache was recreated. i think that could be fixed with the change i commented in #165.

it would be nice to be able to change the cache location by just setting a variable. maybe this could be something that should be configurable.