Shim installation breaks if archive contains other files

[vdice]

I was testing the wasmtime shim and encountered an issue at the installation/containerd config step, preventing a sample app from running. Turns out, the latest containerd-shim-wasmtime/v0.6.0 archive contains other files besides the shim binary, including a .sig and .pem file. (This appears to be the same case for the other shims from https://github.com/containerd/runwasi/releases.)

The containerd configuration ends up looking something like the following, which of course is incorrect:

# RCM runtime config for wasmtime-v1.pem
[plugins."io.containerd.grpc.v1.cri".containerd.runtimes.wasmtime-v1.pem]
runtime_type = "/opt/rcm/bin/containerd-shim-wasmtime-v1.pem"

# RCM runtime config for wasmtime-v1.sig
[plugins."io.containerd.grpc.v1.cri".containerd.runtimes.wasmtime-v1.sig]
runtime_type = "/opt/rcm/bin/containerd-shim-wasmtime-v1.sig"

AC: update download/installation logic to account for multiple files in an archive, installing/configuring only the actual shim binary as intended.