%APPDATA% unset on Windows when using Actions for CI

See logs here: fix var interpolation in workflow · hofstadter-io/hof@072d279 · GitHub

This var is used by Go in the os.UserConfigDir() (os · pkg.go.dev)

Also, any idea why the previous test is so slow? (>10x) It could be related to reading & writing large numbers of files, is that a know issue?