Add (disabled) get-all-crates unit to dev desktops

[Mark-Simulacrum]

This is disabled, but has run successfully (ish, see below) on dev-desktop-us-2.

$ sudo du -h -d 1 /home/crates-io-downloader
1.4G    /home/crates-io-downloader/.rustup
267M    /home/crates-io-downloader/.cargo
20K     /home/crates-io-downloader/.config
8.0K    /home/crates-io-downloader/.ansible
4.8G    /home/crates-io-downloader/crates.io-index
6.3G    /home/crates-io-downloader
$ sudo du -h -d 0 /var/cache/all-crates
385G    /var/cache/all-crates

Running the systemd unit repeatedly exited early due to memory limits -- I think if we want to enable the timer to run this unit on a periodic basis, we'll want to work to either update or replace get-all-crates to not use meaningful memory before we enable this on a continuous basis. It runs incrementally, so I don't think there's actually a hard blocker there, but it feels like there's an opportunity for improvement that should be driven to conclusion.

In the meantime us-2 has a copy in /var/cache/all-crates that is probably useful for folks and we can manually run the unit on an as-needed basis on that host (sudo systemctl start get-all-crates).

cc #t-infra > Put all crates on dev desktops