workflows/artifacts: use stable rustc to install elf2tab

[lschuermann]

This applies the same hack of #486 to work around our current rust-toolchain being too old to build elf2tab.

[jrvanwhy]

Do we still want this with #513 merged? At the moment we're using the MSRV Rust toolchain to install elf2tab in the CI workflow (as well as on developer machines). The artifacts workflow should probably be synchronized with the CI workflow.

I was thinking we'd update the MSRV if a new elf2tab release requires a newer toolchain, but alternatively we could use the nightly toolchain to install elf2tab. Either option avoids adding a third toolchain to libtock-rs's dependencies.

[lschuermann]

I agree we wouldn't want this with #513. It doesn't make sense for us to target an MSRV if we can't use it to build a core piece of our toolchain, such as elf2tab. If elf2tab requires a newer version of Rust, let's bump our MSRV.