Fix PluginResolverTest: isolate the plugin cache. (#8615)
Fix PluginResolverTest: isolate the plugin cache. (#8615)

Previously the attempt made at doing this was failing and the resulting
pollution of ~/.cache/pants/plugins/ was leading to unexpected results.
pantsbuild/pants,wisechengyi/pants,tdyas/pants,tdyas/pants,jsirois/pants,wisechengyi/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,wisechengyi/pants,wisechengyi/pants,wisechengyi/pants,jsirois/pants,wisechengyi/pants,tdyas/pants,tdyas/pants,wisechengyi/pants,benjyw/pants,tdyas/pants,tdyas/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,jsirois/pants,benjyw/pants,tdyas/pants,tdyas/pants,wisechengyi/pants,pantsbuild/pants,tdyas/pants,wisechengyi/pants