Updated mosh upstream to 581bd67c5ad51cb59176472101f6a0b03703e494
This appears necessary to build on macOS Monterey with go 1.18 toolchain. Some paths have changed within mosh. Added a fixup script which needs to be run after mosh is built when building on macOS. Mosh can now be built by invoking the build-mosh script, which should work after the mosh git submodule is initialized.
Also added a Linux CI sanity check.