tweak some git clone commands

[tshepang]

• --depth=1 is more useful for once-off uses, like on ci
• .git postfix on github repo url is not needed

[rustbot]

Thanks for the PR. If you have write access, feel free to merge this PR if it does not need reviews. You can request a review using r? rustc-dev-guide or r? <username>.

[jieyouxu]

--depth=1 can be intentional sometimes, if history doesn't matter for your particular workflow instance.
r? @ZuseZ4 (autodiff workflow, I'm not sure)

[Noratrieb]

wasn't there a problem that depth 1 doesn't work properly with bootstrap?

[jieyouxu]

For rust-lang/rust almost certainly, I was moreso wondering about the enzyme/llvm cases