Miri needs a special pre-built standard library which is normally built on-demand when using cargo miri and cached locally. This standard library needs to match the exact nightly version of the compiler. The rest of Miri is already installed by ./bin/ce_install ....

This PR builds this sysroot for all architectures. If this is too much (due to storage/build time requirements), this could be restricted to the more common architectures.

Because the sysroot/standard library version must exactly match the compiler version, this build includes the full Rust nightly compiler (so there are no conflicts depending on when Miri and Rust nightly are updated on the runners).

Building the sysroot needs the Rust standard library sources, so this PR requires compiler-explorer/infra#1630.

See compiler-explorer/compiler-explorer#2563.