GH-47821: [CI][Release][R] Fix test repository path in release (#47929)

### Rationale for this change

`r/tools/nixlibs.R` assumes that `arrow.repo` ends with `/` but `dev/tasks/macros.jinja` uses `arrow.repo` without the end `/`.

### What changes are included in this PR?

Add missing the end `/` and `/libarrow` sub directory.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #47821

Authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: kou

dev/tasks/macros.jinja

@@ -286,7 +286,7 @@ env:
     run: |
       profile_path <- file.path(getwd(), ".Rprofile")
       repo <- paste0("file://", getwd(), "/repo")
-      str <- paste0("options(arrow.repo = '", repo, "' )")
+      str <- paste0("options(arrow.repo = '", repo, "/libarrow/' )")
       print(str)
       write(str, file = profile_path, append = TRUE)
       str <- paste0("options(arrow.dev_repo = '", repo, "' )")