sanity_checks: add tip about using old format

[meator]

I spent a lot of time trying to follow this error message's advice to run tools/format.py without success. It took me a while to figure out that newer versions of Meson modified its formatting rules, so a file that was correctly formatted with Meson 1.6.1 (to give an example) will be considered unformatted by Meson 1.9.0rc2 (or whatever the runners use now). 1.6.1 isn't even that old.

The wording may need to be changed, this PR is more of a proposal.

[bgilbert]

Perhaps format.py should install the latest Meson release into a virtualenv and run it from there, either always or only if the system's Meson is too old.

[bgilbert]

#2325 changes format.py to run Meson from a virtualenv it maintains with the latest prerelease.

[meator]

Wow, thanks @bgilbert!