Make CARGO_HOME optionally writable

[skius]

We're running into issues with rustwide in qrates when incremental compilation is enabled:

--snip--
14:08:57 [INFO] [stdout] 948fb9cd0c36f8264ca53a8d474817f66d2eb5db6ca5eee8a9c271c2887d41c4
14:08:57 [INFO] running `Command { std: "docker" "start" "-a" "948fb9cd0c36f8264ca53a8d474817f66d2eb5db6ca5eee8a9c271c2887d41c4", kill_on_drop: false }`
14:08:57 [INFO] [stderr]     Checking powerfmt v0.2.0
14:08:57 [INFO] [stderr]     Checking num-conv v0.1.0
14:08:57 [INFO] [stderr]     Checking time-core v0.1.2
14:08:57 [INFO] [stderr] error: could not create incremental compilation crate directory `incremental/powerfmt-33plc6te6e1kj`: Read-only file system (os error 30)
14:08:57 [INFO] [stderr]
14:08:57 [INFO] [stderr] error: could not create incremental compilation crate directory `incremental/time_core-0itg8lzr2ib0v`: Read-only file system (os error 30)
14:08:57 [INFO] [stderr]
14:08:57 [INFO] [stderr] error: could not create incremental compilation crate directory `incremental/num_conv-2hmvxbsyjvjzr`: Read-only file system (os error 30)
14:08:57 [INFO] [stderr]
14:08:57 [INFO] [stderr] error: could not compile `time-core` (lib) due to 1 previous error
14:08:57 [INFO] [stderr] warning: build failed, waiting for other jobs to finish...
14:08:57 [INFO] [stderr] error: could not compile `powerfmt` (lib) due to 1 previous error
14:08:58 [INFO] [stderr] error: could not compile `num-conv` (lib) due to 1 previous error
--snip--

This PR adds an option to Command to allow mounting the $CARGO_HOME directory as read/write, which resolves our issue.

I am unsure if this is the way to solve the issue, so I'm opening this PR in draft state. I would be happy as well if there was a way to set the incremental directory to something ephemeral inside the vfs and keeping $CARGO_HOME read-only. Our project is only interested in some side-effects of incremental compilation in rustc (computing HIR hashes), not the incremental nature itself.

Thanks!

[skius]

The CI failure seems unrelated:

 error: a method with this name may be added to the standard library in the future
  --> src/utils.rs:45:18
   |
45 |     let _ = file.unlock();
   |                  ^^^^^^
   |
   = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
   = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
   = help: call with fully qualified syntax `fs2::FileExt::unlock(...)` to keep using the current method
   = note: `-D unstable-name-collisions` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unstable_name_collisions)]`

error: could not compile `rustwide` (lib) due to 1 previous error