Switch to using a memfs instead of an osfs

[aiyengar2]

Currently, charts-build-scripts uses a osfs:

charts-build-scripts/pkg/filesystem/filesystem.go

Line 24 in 2901446

return osfs.New(path)

However, ideally we should be making loading the current directory into a memfs, making all calls in memory, and then only modifying the underlying filesystem after executing all operations.

https://github.com/go-git/go-billy/blob/205ba6585ca03bd60e304f7fa0b270232ba1d707/memfs/memory.go#L28-L31