Automatic toolchain upgrade fails with argument-list-too-long

[tautschnig]

https://github.com/model-checking/kani/actions/runs/15432529867/job/43432847335 failed as the git_log is exported as an environment variable, yet that log appears too long for other commands to make use of.

We should try to communicate it via a file.

[zhassan-aws]

I examined the failed run more closely and found out that the git log is actually including commits that are not between the current toolchain commit and the next toolchain commit, e.g.

  https://github.com/rust-lang/rust/commit/178fa2a806 travis CI
  https://github.com/rust-lang/rust/commit/51db348a3f README: link back to rust-lang/rust issue
  https://github.com/rust-lang/rust/commit/773fd1a707 initial commit

where "initial comimit" is from Aug 7, 2016!!

https://github.com/model-checking/kani/actions/runs/15432529867/job/43432847335#step:5:2602

The fix is to pass --ancestry-path to the git log command.