This fixes a bug in the `GlobalCacheTracker::git_checkout_all` function
which was using the wrong SQL column name. This function belongs to the
set of of functions only used for testing, and this particular function
was not yet used.

This adds the `Execs::args` method which forwards to
`ProcessBuilder::args` to specify multiple command arguments at once.

This adds the `requires_rustup_stable` option to the cargo_test macro to
require `rustup` to exist, and for the `stable` toolchain to be
installed.

It is required that stable be installed in Cargo's CI. On a developer
machine, the tests will be ignored if rustup is not installed. It will
also be ignored on rust-lang/rust's CI since it does not have rustup.

This adds some tests to ensure that the database used in the global
cache tracker stays compatible across versions. These tests work by
using rustup to run both the current cargo and the stable cargo, and
verifying that when switching versions, everything works as expected.

On Windows, rustup has an issue with recursive cargo invocations. This
commit can be removed once
rust-lang/rustup#3036 is resolved.

Cargo likes to modify PATH, which circumvents the ability to choose the
correct "cargo" executable to run on Windows (because Windows uses PATH
for both binary and shared library searching).