Commit 9cc2b7cd2f regressed the `--source-feed`
option so that it no longer accepted relative paths. Because the option is now
saved to the temp project file, any relative paths specified by the
`--source-feed` option were made relative to the temp project path and not from
the current working directory of where dotnet was run.
The fix is to use `Path.GetFullPath` of the `--source-feed` option, provided
the option specified was not an absolute URI.
Fixes#9132.
b9597913dc