Less toolchain reinstalls in manylinux/musllinux

[andrius-puksta-sensmetry]

maturin-action always performs needless Rust toolchain installs for builds on manylinux/musllinux. As we observed from CI logs, the steps are:

• (only on musllinux) remove existing stable toolchain (example CI run)
• install latest stable
• (if rust-toolchain.toml is present) install toolchain specified in rust-toolchain.toml. In our case, it is the latest stable version 1.92, but since "1.92" and "stable" are from different channels, rustup installs another copy of the same.

This is wasteful. Ideally, existing toolchain should be used if possible, if not, a single install of the toolchain should be performed, taking into account rust-toolchain.toml.

[andrius-puksta-sensmetry]

Related rustup issue: rust-lang/rustup#4663