Enable DOTNET_INSTALL_DIR environment variable

In scope of this release the DOTNET_INSTALL_DIR environment variable was enabled to add the ability to change the installation directory for .NET SDK #329. The DOTNET_INSTALL_DIR can be helpful when access to the default installation directory, for instance, on self-hosted runners, is denied.

Example of usage:

build:
  runs-on: self-hosted
  env:
    DOTNET_INSTALL_DIR: "path/to/directory"
  steps:
    - uses: actions/setup-dotnet@v3
      with:
        dotnet-version: '3.1.x'