azure: Run tool builds whenever a tool submodule is updated

[alexcrichton]

This issue is meant to track this TODO annotation where on Travis we currently run the x86_64-gnu-tools builder whenever the commit message looks like it updates a submodule.

We should enable something similar for Azure Pipelines where whenever a PR is detected as updating a submodule (either via actually looking at the git history or at the git commit message) then we schedule the builder on the PR. The purpose of this PR build is to help weed out errors even sooner than before where we're just running a smoke tests on Linux on each PR.