Consider larger disk space for CI executors

[rpoyner-tri]

Is your feature request related to a problem? Please describe.
See also pr #23258 and issue #21836. Motivated by mixed rust and c++ programs breaking line-by-line debug symbols, we are experimenting with the mold linker to solve debug symbol problems specifically.

The current obstacle is that (even with symbol compression), the total build output is larger. It sometimes barely fits in our current CI disk space, but often does not.

Describe the solution you'd like
Current CI disk space appears to be ~400G. Consider enlarging this allocation by 50% to ~600G. Explore how much more this would cost month-to-month.

Describe alternatives you've considered
I've already applied symbol compression (helpful) and considered lopping off build tasks for the debug builds only (very difficult to make a difference and hard to maintain).

Additional context
See the discussion of disk space in pr #23258 .

[rpoyner-tri]

Assigning to @BetsyMcPhail for further triage and delegation.

[rpoyner-tri]

From discussion in kitware meeting, we should investigate:

• split-dwarf
• dynamic linking
  to further reduce the disk space needs of debug builds.