Isolate plugins

[danielrbradley]

Disable plugin downloads by default and add option to install specific plugins.

Generally when running tests, you want to have a controlled environment. Therefore, plugins should not be auto-downloaded during the test but be specified ahead of time.

This could be done by using the PULUMI_DISABLE_AUTOMATIC_PLUGIN_ACQUISITION setting by default with options to not disable it, and to install specific plugin versions (or latest) during the test setup phase.

Additionally, we should consider using an isolated plugins directory for each test to ensure tests don't interfere with each other by downloading multiple versions then accidentally using the wrong version.

This was created off the back of mis-typing a provider's name when wanting to attach but the test continuing and downloading the latest published version instead: pulumi/pulumi-azure-native#3760