When installed via snap there is a fpcalc included in the package. The path to this is something like "/snap/picard/221/usr/bin/fpcalc". It works directly after install, but if there was an update this path becomes invalid.

What would work is the stable path "/snap/picard/current/usr/bin/fpcalc"

We already have some special handling for finding fpcalc when installed as part of pyinstaller package (on macOS and Windows). Maybe we need to treat this specially as well. One option might be a command line parameter or environment variable to specify (default) fpcalc location.