Log disk usage in CI

[captbaritone]

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Trying to debug CI failures caused by running out of disk.

Running Rust tests on Ubuntu is what's failing reliability. Specifically, it fails when compiling the tests. Right before I run tests (which compiles tests as a side effect). The disk usage reported is:

Filesystem      Size  Used Avail Use% Mounted on
/dev/root        73G   55G   18G  76% /
tmpfs           7.9G  172K  7.9G   1% /dev/shm
tmpfs           3.2G  1.1M  3.2G   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
/dev/sda15      105M  6.1M   99M   6% /boot/efi
/dev/sdb1        74G  4.1G   66G   6% /mnt
tmpfs           1.6G   12K  1.6G   1% /run/user/1001

So we should still have 18BG to play with on the main disk, assuming that's where compiler artifacts will be written?

If I run CARGO_TARGET_DIR=~/tmp/cargo-test cargo test --features vendored on my local machine (mac). Only 4.2 G of stuff is written to ~/tmp/cargo-test, which seems like it shouldn't be a problem given we have 18G free...