fix(remote): define special variables behavior

Issue #2267 — Define semantics of file-path special variables when the
Taskfile is loaded from a remote source (HTTP/HTTPS/Git):

- TASKFILE / ROOT_TASKFILE: raw URL (fixes the broken `https:/...`
  caused by filepath.Join collapsing the double slash)
- TASKFILE_DIR / ROOT_DIR: empty string — a DIR variable cannot point
  to a URL
- TASK_DIR: resolved against USER_WORKING_DIR

Export taskfile.IsRemoteEntrypoint so the compiler can dispatch on the
nature of the entrypoint without relying on `c.Dir == ""` (a side
effect of the remote path).

taskfile/node_git.go

@@ -188,7 +188,7 @@ func (node *GitNode) ReadContext(ctx context.Context) ([]byte, error) {
 
 func (node *GitNode) ResolveEntrypoint(entrypoint string) (string, error) {
 	// If the file is remote, we don't need to resolve the path
-	if isRemoteEntrypoint(entrypoint) {
+	if IsRemoteEntrypoint(entrypoint) {
 		return entrypoint, nil
 	}