fix Registry::args for plugins loaded with --extra-plugins

[oli-obk]

r? @Manishearth

[Manishearth]

@bors r+

[bors]

📌 Commit b33be5d has been approved by Manishearth

[bors]

☔ The latest upstream changes (presumably #33228) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

rebased

[oli-obk]

this is ready to go

[Manishearth]

@bors r+

[bors]

📌 Commit 46de0fa has been approved by Manishearth

[bors]

⌛ Testing commit 46de0fa with merge 0352866...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild