Explicitly adding a mount pair for file in the user's home directory

[bd-tkoolen]

In an attempt to increase hermeticity, we're interested in enabling --experimental_use_hermetic_linux_sandbox and then explicitly adding --sandbox_add_mount_pairs. However, we need to make the user's ~/.netrc file available in the sandbox. Is there any way to do that in a way that doesn't hardcode the absolute path of the user's home directory? We don't want to mount all of /home.