test: set GIT_SSH_COMMAND in V2 remote-fetch test to override env SSH (#38)

The test fakes GIT_SSH to intercept git fetch over SSH, but the
host environment exports GIT_SSH_COMMAND (which takes priority over
GIT_SSH), so git was using the real ssh binary and getting a real
GitHub 404. Set GIT_SSH_COMMAND to the same fake script so the test
is deterministic across environments.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

Author: Patel230

cli/explain_test.go

@@ -1962,6 +1962,7 @@ esac
 exec git-upload-pack "$repo"
 `), 0o755))
 	t.Setenv("GIT_SSH", sshScript)
+	t.Setenv("GIT_SSH_COMMAND", sshScript) // GIT_SSH_COMMAND takes priority over GIT_SSH on systems where it's set globally.
 	t.Setenv("CHECKPOINT_REPO", checkpointDir)
 
 	require.NoError(t, os.MkdirAll(filepath.Join(localDir, ".trace"), 0o755))