ci: support git-cache-rs [backport 2024.01]

[MrKevinWeiss]

Backport of #20311

Contribution description

This PR adds support for the Rust version of git-cache.

The shell version that we've integrated in dist/tools/git is quite a mess, barely maintainable. But most importantly, it is slow, and gets slower over time, due to the way the cache stores tags. Also, it doesn't support sparse checkouts. Crude measurements show that for our CI builds, for cortex-m builds 20-50% of time spent on each built are spent on checking out "pkg/cmsis".

git-cache-rs' syntax is compatible with regular git clone to the point where any git clone ... command that doesn't work with just changing to git cache clone is considered a bug.
Unfortunately that requires some changes. This PR introduces git-cache-rs support as fully optional, it'll only be used if GIT_CACHE_RS is set to the binary name. The legacy built-in shell git-cache should continue working like before.

git-cache-rs works a bit differently than the shell git-cache: instead of having one large bare cache repository that adds repos as remote, git-cache-rs just stores a "--mirror" repository for each cached repo. This is much simpler, although it might use a bit more space.

git-cache-rs supports sparse checkouts, which has been implemented here for pkg/cmsis. I've updated dwq and the murdock container, and am currently testing those changes on ci-staging.riot-os.org.

Testing procedure

I've been testing this PR together with the CI support in RIOT-OS/riotdocker#240 for a day or so on https://ci-staging.riot-os.org/.

Issues/PRs references

[riot-ci]

Murdock results

✔️ PASSED

a658c0a pkg: ensure "dynamic sparse paths" are up-to-date

Success	Failures	Total	Runtime
8629	0	8629	08m:33s

Artifacts

• Documentation preview

[kaspar030]

ACK.

[kaspar030]

Is this waiting for bors!?