Allow even more dynamic instrumentation of pwd via os.dynamicPwdFunction

[lihaoyi]

Follow up to #298, which it turns out was not flexible enough to do what we need in Mill. Mill uses references to the pwd to create the .dest folder lazily, thus we cannot just assign a value to the os.pwd but actually need to assign a lambda that can be replaced to implement the lazy evaluation

I pulled the trigger too quickly on releasing 0.10.6 with os.dynamicPwd, so this one has to be named differently. I chose os.dynamicPwdFunction and will make sure to test it with an unstable release before releasing a stable tag