Improve compare_fedora_versions so that it restricts the dependencies it looks for to linux ones

[mulkieran]

stratisd has no non-linux dependencies, but other packages that I have been running it on are truly multi-platform.

I have not figured out how to get "cargo metadata" to do this, although it has a --filter-platform option which looked promising.

[mulkieran]

Unfortunately, the --filter-platform will be cumbersome to use, as we would have to list all target triples or use wildcards. If we use a wildcard like ``"*-unknown-linux-gnu"``` the call will fail on Fedora systems. We could use rustc --print target-list to filter out what are supported on the platform on which the command is run, which might work. This seems like not worth the trouble at this time.

[mulkieran]

We can maybe make this dependent on cargo-vendor-filterer output. That is, if the fedora conflict that was reported had been eliminated from the vendor-filterer vendor tarfile, then we should not have to worry about it in Fedora.