This commit implements milestone 2.1 for the gitannex subcommand:
https://github.com/rclone/rclone/issues/7625#issuecomment-1951403856
This rewrite makes a few improvements over the old shell script:
(1) It no longer uses the system's rclone.conf. Now, it writes the
rclone.conf file in an ephemeral directory.
(2) It no longer makes any assumptions about the contents of /tmp.
However, it now assumes that an rclone built from the HEAD commit is on
the PATH. It makes a best-effort attempt to verify this assumption, but
I'm not sure it's bulletproof.
I'm hoping that writing this in Go will enable more cross-platform
support in the future, but for now we're still restricted to Unixy
systems due to reliance on the HOME environment variable.
Issue #7625
