Make sure cli is installed in .libPaths()[2]

[jennybc]

I don't understand how this test can be brittle and yet it has failed once: https://github.com/r-lib/devtools/actions/runs/25173878432/job/73800519118. It failed on retry as well and notably it's a cache miss re: setting up R dependencies:

Cache not found for input keys: macOS Sequoia 15.7.4-R version 4.6.0 (2026-04-24)-aarch64-1-46c104b8b7bd9f1e9303c0a9f6f871cc48555d9b4498060900dd0f901bfac1c7, macOS Sequoia 15.7.4-R version 4.6.0 (2026-04-24)-aarch64-1-

══ Failed tests ════════════════════════════════════════════════════════════════
── Failure ('test-install.R:37:3'): install() doesn't reinstall deps already in non-primary libpath ──
Expected `installed` to equal "testInstallWithDeps".
Differences:
`actual`:   "cli" "testInstallWithDeps"
`expected`:       "testInstallWithDeps"


[ FAIL 1 | WARN 0 | SKIP 7 | PASS 139 ]

I feel like it could be some buglet in pak? Or very subtle thing about our GHA config? But neither is in scope for devtools, so let's just harden the test.

Earlier macos-latest GHA runs succeeded with practically the same devtools code and I see:

Cache hit for: macOS Sequoia 15.7.4-R version 4.6.0 (2026-04-24)-1-a3e6a6a4321bed33fbcd758f3191ac818942771c229fd4b72fc1a86a3bb9b130
...
Cache restored successfully
Cache restored from key: macOS Sequoia 15.7.4-R version 4.6.0 (2026-04-24)-1-a3e6a6a4321bed33fbcd758f3191ac818942771c229fd4b72fc1a86a3bb9b130

[jennybc]

I now see success on macos-latest even though there's still a cache miss. So that's encouraging:

Cache not found for input keys: macOS Sequoia 15.7.4-R version 4.6.0 (2026-04-24)-aarch64-1-46c104b8b7bd9f1e9303c0a9f6f871cc48555d9b4498060900dd0f901bfac1c7, macOS Sequoia 15.7.4-R version 4.6.0 (2026-04-24)-aarch64-1-
...

ℹ Installing lockfile '.github/pkg.lock'
   
  → Will install 112 packages.