cmd/gitannex: Replace e2e test script with Go test

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

.github/workflows/build.yml

@@ -124,7 +124,7 @@ jobs:
           sudo modprobe fuse
           sudo chmod 666 /dev/fuse
           sudo chown root:$USER /etc/fuse.conf
-          sudo apt-get install fuse3 libfuse-dev rpm pkg-config
+          sudo apt-get install fuse3 libfuse-dev rpm pkg-config git-annex
         if: matrix.os == 'ubuntu-latest'
 
       - name: Install Libraries on macOS
@@ -137,6 +137,7 @@ jobs:
           brew untap --force homebrew/cask
           brew update
           brew install --cask macfuse
+          brew install git-annex
         if: matrix.os == 'macos-11'
 
       - name: Install Libraries on Windows