Turned out I had neglected the fact that I had installed a 64 bit kernel on my Pi when installing the nspawn 64 bit shell. So although the Pi was still running a 32 bit user land the kernel identified as 64 bit which confused the Rust installer.
If one follows the Rust installation instructions here: https://www.rust-lang.org/tools/install the resulting Rust installation will not run on a Pi4 with a 32 bit Pi OS. At least not if one just does the default install. Which is odd, I'm sure I had it working in the spring time. Seems the installer decides the Pi 4 is a 64 bit machine and it all goes wrong.
To get it to work:
1) Download the installer as per the link above:
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$ curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
3) When prompted for "Default host triple?" enter "arm-unknown-linux-gnueabihf"
Other options are up to you, I selected "nightly" for the default toolchain, "complete" for the profile and "y" to update my PATH. And finally 1) to kick off the installation.
After that "cargo" works fine again.