Use Path.join() to join tmp directory and briefly-nnnnnn subdirectory

[srgpqt]

Without Path.join(), Briefly tries to create files like /tmpbriefly-123, missing a / in the path

[warmwaffles]

@bruce or @benwilson512 I think this can be merged

[mcrumm]

🍬 🍬 🍬 🍬 🍬

[warmwaffles]

This works for me @mcrumm thanks a ton