Exclude the target directory from backups using CACHEDIR.TAG

Exclude the target directory from backups using CACHEDIR.TAG

This patch follows the lead of #4386 (which excludes target directories
from Time Machine backups) and is motived by the same reasons listen
in #3884. CACHEDIR.TAG is an OS-independent mechanism supported by Borg,
restic, GNU Tar and other backup/archiving solutions.

See https://bford.info/cachedir/ for more information about the
specification. This has been discussed in Rust Internals earlier this
year[1] and it seems like it's an uncontroversial improvement so I went
ahead with the patch.

[1] https://internals.rust-lang.org/t/pre-rfc-put-cachedir-tag-into-target/12262/11