OpenSSH for Windows hands the SSH command argument to the login shell
wrapped in double quotes, which expanded the `$`/`$(...)` in Herdr's POSIX
discovery and API bridge scripts before `/bin/sh` ran them. A saved machine
whose default shell is WSL bash is detected as POSIX, so `herdr --machine`
then failed with `remote binary discovery failed`.
Deliver the discovery script on stdin (`/bin/sh -s`) and base64-encode the
cached POSIX API bridge command so the whole script, including any executable
path, stays out of the command argument. The decoded bridge reads the API
payload from fd 3 so decoding the script cannot consume it.
refs #5132
JJ Liebig
Can Celik